feat(runtime,io): driver-enqueues + park/wake idle path — retire the wake pipe

The swap (RFC 018). Schedulers no longer sleep on a shared level-triggered
wake pipe — the herd source that made the default 8-thread config 7x
slower than 2 threads (E1). They park on per-thread futex parkers via the
coordination layer; IO backends become producers behind a two-call
contract (make runnable, then the enqueue tail wakes exactly one parked
scheduler).

Deleted: the drain lock and the one-winner phase-1 drain; the shared
completions VecDeque; the wake pipe fds, poll_wake, drain_wake_pipe,
wake_scheduler, the FdReady/Blocking Completion enum; the 100us idle nap;
the per-pop io.lock liveness read; io.rs's as_millis timeout truncation.

Added:
- enqueue wake tail (fixes the silent enqueue): wake_one_if_idle, a fence
  + one Relaxed mask load when everyone is busy — the pure-compute hot
  path pays almost nothing.
- driver-enqueues: the pool thread stashes its result in the slot,
  decrements io_outstanding, unparks; the epoll thread removes+DELs the
  waiter under the waiters lock and unparks. Both reach the runtime via a
  Weak (no Arc cycle). The waiters map moves behind its own Arc<Mutex> so
  the epoll thread never takes the runtime io lock (teardown holds it
  while joining that thread).
- io_outstanding / io_fd_waiters atomics: the termination verdict reads
  two atomics instead of taking io.lock on every pop.
- timekeeper idle path: at most one parked scheduler holds the timer
  deadline (an expiry wakes one, not a herd); everyone else parks
  indefinitely and is woken by the enqueue tail.
- busy-path timer due-check (ratified design point (a)): under saturation
  nobody parks and no timekeeper exists, yet due timers must still fire —
  one Relaxed load of the earliest-deadline snapshot per loop, clock read
  only when a timer is armed. Maintained under the timers mutex.
- chain rule: a scheduler that pops with more work queued and a sibling
  parked wakes one, so surplus runs in parallel rather than behind it.

tests/park_wake.rs pins the two new observable properties: timers fire
under full scheduler saturation, and sub-ms sleeps are prompt (the
as_millis truncation regression). Full suite + all loom models green;
clippy --lib clean.

.githooks/pre-commit

@@ -0,0 +1,24 @@
+#!/bin/sh
+# smarm pre-commit gate: clippy the library (src/) with warnings as errors.
+# unwrap_used / expect_used are denied (Cargo.toml [lints.clippy]): library
+# code must not hide a panic behind unwrap/expect. Tests/examples are not gated.
+#
+# Toolchain resolution: prefer an installed cargo-clippy; on machines whose
+# rust comes without the clippy component (e.g. NixOS home-manager), fall
+# back to an ephemeral nix-shell toolchain. The fallback uses its own target
+# dir (target/clippy) because the shell's rustc version may differ from the
+# default toolchain's — mixed-compiler artifacts in one target dir are an
+# E0514 hard error. MSRV (Cargo.toml rust-version) keeps the older shell
+# toolchain a legitimate gate.
+set -eu
+[ -f "$HOME/.cargo/env" ] && . "$HOME/.cargo/env"
+cd "$(git rev-parse --show-toplevel)"
+if cargo clippy --version >/dev/null 2>&1; then
+    cargo clippy --lib -- -D warnings
+elif command -v nix-shell >/dev/null 2>&1; then
+    nix-shell -p clippy -p cargo -p rustc \
+        --run 'CARGO_TARGET_DIR=target/clippy cargo clippy --lib -- -D warnings'
+else
+    echo "pre-commit: cargo clippy unavailable and no nix-shell fallback" >&2
+    exit 1
+fi

.gitignore

@@ -4,3 +4,4 @@ smarm_trace.json
 /bench_results/
 __pycache__/
 *.pyc
+profile.coz

Cargo.toml

@@ -7,9 +7,22 @@ rust-version = "1.95"
 [lints.rust]
 unexpected_cfgs = { level = "warn", check-cfg = ["cfg(loom)"] }
 
+[lints.clippy]
+# Library code must never hide a panic behind unwrap/expect. Both are denied; an
+# intentional panic is written explicitly as `match { Err(e) => panic!(..) }`.
+# panic!/unreachable! are deliberately left un-linted as the blessed explicit
+# form. Enforced on the library target only (`cargo clippy --lib`); tests and
+# examples unwrap freely and are not gated.
+unwrap_used = "deny"
+expect_used = "deny"
+
 [features]
 default = ["rq-mutex"]
 smarm-trace = []
+# RFC 007: native causal profiling. Zero cost when off (cf. smarm-trace): the
+# hook in `maybe_preempt` and the resume-path fast-forward compile away; the
+# two Slot ledger fields exist regardless and stay 0 (budget_cycles precedent).
+smarm-causal = []
 # RFC 016 Chunk 2: cycle-accurate per-actor time-budget accounting. Off by
 # default — it costs two extra RDTSC reads per actor resume on the hot path
 # (D6). The `ActorInfo.budget_cycles` field exists regardless; it just stays 0
@@ -80,3 +93,15 @@ harness = false
 [[example]]
 name = "observer"
 required-features = ["observer"]
+
+[[example]]
+name = "causal_pipeline"
+required-features = ["smarm-causal"]
+
+[[example]]
+name = "causal_attrib_probe"
+required-features = ["smarm-causal"]
+
+[[example]]
+name = "causal_probe"
+required-features = ["smarm-causal"]

examples/causal_attrib_probe.rs

@@ -0,0 +1,198 @@
+//! Attribution-efficiency probe (RFC 007 follow-up).
+//!
+//! Original hypothesis: the ~4pt impact shortfall on the 24-core
+//! validation (+29.3/+83.5 vs theoretical +33/+100) is a constant
+//! attribution efficiency eff ≈ 0.91 from site-exit tail truncation.
+//! The guard-drop flush closed that leak, yet eff held at ~0.93 —
+//! RESOLVED (2026-07-13 sweep): the residual is runnable off-CPU time
+//! inside the site (~4.9 slice-expiry yields/entry x ~5.6µs runqueue
+//! wait), wall time the ground truth below counts but on-CPU
+//! attribution correctly skips. The offcpu audit bucket now counts it;
+//! `eff+offcpu` printed per window should sit at ~1.00 — the
+//! closed-books check.
+//!
+//! Measurement: same pipeline as `causal_pipeline`, but the `reserve`
+//! actor also measures its raw in-site time directly (rdtsc at guard
+//! enter/exit) and counts site entries. For each experiment window at
+//! pct%:
+//!
+//!     eff = (Δglobal_delay / (pct/100)) / Δin_site_cycles
+//!
+//! and the missing time per site entry localizes the leak:
+//!
+//!     tail_us/entry = (Δin_site − Δglobal_delay/(pct/100)) / Δentries
+//!
+//! A constant eff across 25/50% with tail/entry in the tens of µs
+//! localizes a per-entry mechanism; `eff+offcpu` ≈ 1.00 confirms the
+//! runnable-gap account and rules out any remaining silent loss.
+//!
+//! Run: cargo run --release --example causal_attrib_probe --features smarm-causal
+
+use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
+use std::sync::Arc;
+use std::time::{Duration, Instant};
+
+fn rdtsc() -> u64 {
+    // x86_64 only — same clock the ledger uses.
+    unsafe { core::arch::x86_64::_rdtsc() }
+}
+
+/// Same fixed-work loop as causal_pipeline (dependent LCG, preemptible).
+fn work_iters(iters: u64) {
+    let mut acc = 0x2545_f491_4f6c_dd1du64;
+    let mut i = 0u64;
+    while i < iters {
+        let chunk_end = (i + 256).min(iters);
+        while i < chunk_end {
+            acc = acc.wrapping_mul(6364136223846793005).wrapping_add(i);
+            i += 1;
+        }
+        std::hint::black_box(acc);
+        smarm::check!();
+    }
+}
+
+fn calibrate_iters_per_us() -> u64 {
+    let n = 8_000_000u64;
+    let t = Instant::now();
+    work_iters(n);
+    (n / (t.elapsed().as_micros().max(1) as u64)).max(1)
+}
+
+static IN_SITE_CYCLES: AtomicU64 = AtomicU64::new(0);
+static SITE_ENTRIES: AtomicU64 = AtomicU64::new(0);
+
+fn main() {
+    let per_us = calibrate_iters_per_us();
+    println!("calibration: {per_us} work iters/µs");
+    let work_us = move |us: u64| work_iters(us * per_us);
+
+    let cores = std::thread::available_parallelism().map(|n| n.get()).unwrap_or(1);
+    println!("cores: {cores}");
+    if cores < 4 {
+        println!("probe: SKIPPED (needs the stages in parallel)");
+        return;
+    }
+
+    smarm::init(smarm::Config::default()).run(move || {
+        let stop = Arc::new(AtomicBool::new(false));
+        let (tx_ab, rx_ab) = smarm::channel::<u64>();
+        let (tx_bc, rx_bc) = smarm::channel::<u64>();
+
+        let stop_p = stop.clone();
+        let producer = smarm::spawn(move || {
+            let mut i = 0u64;
+            while !stop_p.load(Ordering::Relaxed) {
+                {
+                    let _g = smarm::causal_site!("serialize");
+                    work_us(200);
+                }
+                if tx_ab.send(i).is_err() {
+                    break;
+                }
+                i += 1;
+            }
+        });
+
+        // Reserve: the target — instrumented with ground-truth in-site time.
+        let reserve = smarm::spawn(move || {
+            while let Ok(item) = rx_ab.recv() {
+                {
+                    let t0 = rdtsc();
+                    let _g = smarm::causal_site!("reserve");
+                    work_us(400);
+                    // Measured before guard drop: exactly the span the
+                    // ledger should be attributing.
+                    IN_SITE_CYCLES.fetch_add(rdtsc().saturating_sub(t0), Ordering::Relaxed);
+                    SITE_ENTRIES.fetch_add(1, Ordering::Relaxed);
+                }
+                if tx_bc.send(item).is_err() {
+                    break;
+                }
+            }
+        });
+
+        let notify = smarm::spawn(move || {
+            while rx_bc.recv().is_ok() {
+                {
+                    let _g = smarm::causal_site!("notify");
+                    work_us(50);
+                }
+                smarm::progress!("orders-processed");
+            }
+        });
+
+        let stop_bg = stop.clone();
+        let background = smarm::spawn(move || {
+            while !stop_bg.load(Ordering::Relaxed) {
+                let _g = smarm::causal_site!("background-compaction");
+                work_us(500);
+            }
+        });
+
+        smarm::sleep(Duration::from_millis(300));
+
+        let hz = smarm::causal::tsc_hz();
+        println!("tsc_hz: {:.3} GHz", hz / 1e9);
+
+        // Manual windows so ledger/ground-truth snapshots align exactly.
+        for &pct in &[25u32, 50, 50, 25] {
+            let g0 = smarm::causal::global_delay_cycles();
+            let s0 = IN_SITE_CYCLES.load(Ordering::Relaxed);
+            let e0 = SITE_ENTRIES.load(Ordering::Relaxed);
+            let a0 = smarm::causal::ledger_counters();
+            smarm::causal::begin_experiment_for_test("reserve", pct);
+            smarm::sleep(Duration::from_millis(1000));
+            smarm::causal::end_experiment_for_test();
+            let audit = smarm::causal::ledger_counters().delta_since(&a0);
+            let injected = smarm::causal::global_delay_cycles() - g0;
+            let in_site = IN_SITE_CYCLES.load(Ordering::Relaxed) - s0;
+            let entries = SITE_ENTRIES.load(Ordering::Relaxed) - e0;
+
+            let attributed = injected as f64 / (pct as f64 / 100.0);
+            let eff = attributed / in_site as f64;
+            // Books-closure check: add back the runnable off-CPU gaps the
+            // audit counted (delta terms -> raw via /pct) — should be ~1.00.
+            let eff_closed = (injected as f64 + audit.offcpu_in_site_cycles as f64)
+                / (pct as f64 / 100.0)
+                / in_site as f64;
+            let missing = in_site as f64 - attributed;
+            let tail_us = if entries > 0 {
+                missing / entries as f64 / hz * 1e6
+            } else {
+                f64::NAN
+            };
+            println!(
+                "pct {pct:>2}%  in_site {:>8.1}ms  attributed {:>8.1}ms  eff {eff:.3}  eff+offcpu {eff_closed:.3}  entries {entries}  missing/entry {tail_us:.1}µs",
+                in_site as f64 / hz * 1e3,
+                attributed / hz * 1e3,
+            );
+            // RFC 007 deficit hunt: name the losses. Drop/discard columns are
+            // in would-be delta terms — divide by pct/100 to compare with the
+            // missing attribution above.
+            let ms = |c: u64| c as f64 / hz * 1e3;
+            println!(
+                "        audit: absorbed {:>7.1}ms  forgiven {:>6.1}ms  drop park {:>5.2}ms/{:<5} yield {:>5.2}ms/{:<5} offcpu {:>6.2}ms/{:<5} discard >max {:>5.2}ms/{:<3} unarmed {}",
+                ms(audit.spin_absorbed_cycles),
+                ms(audit.park_forgiven_cycles),
+                ms(audit.drop_park_cycles),
+                audit.drop_park_n,
+                ms(audit.drop_yield_cycles),
+                audit.drop_yield_n,
+                ms(audit.offcpu_in_site_cycles),
+                audit.offcpu_in_site_n,
+                ms(audit.discard_overmax_cycles),
+                audit.discard_overmax_n,
+                audit.discard_unarmed_n
+            );
+            smarm::sleep(Duration::from_millis(150));
+        }
+
+        stop.store(true, Ordering::Relaxed);
+        producer.join().unwrap();
+        reserve.join().unwrap();
+        notify.join().unwrap();
+        background.join().unwrap();
+        println!("probe: DONE");
+    });
+}

examples/causal_pipeline.rs

@@ -0,0 +1,296 @@
+//! Causal-profiling demo (RFC 007): a pipeline where conventional profiling
+//! lies and causal profiling doesn't.
+//!
+//!   producer --(serialize ~200µs/item)--> reserve --(~400µs/item)--> notify
+//!   background: an actor burning CPU constantly, fully off the critical path
+//!
+//! `reserve` is the true bottleneck. `serialize` is hot but overlapped with
+//! `reserve`'s backlog, and `background` is the hottest code in the process
+//! while contributing nothing to throughput. A cycle profiler ranks them
+//! background > reserve ≈ 2×serialize; the causal report instead shows
+//! throughput responding to virtual speedups of `reserve` and (near-)ignoring
+//! `serialize` and `background`.
+//!
+//! Stage cost is fixed *work* (a calibrated arithmetic loop), not fixed wall
+//! time. This matters: a timed busy-wait absorbs injected causal delay into
+//! its own budget and finishes on schedule regardless, making every
+//! experiment read as a no-op (found live on a 24-core run: dead-flat
+//! deltas). Real workloads are work-shaped, so the demo must be too.
+//!
+//! Run:
+//!   cargo run --release --example causal_pipeline --features smarm-causal
+//!
+//! Modes (`SMARM_CAUSAL_MODE`), for probing what the guard placement leaves
+//! out of the measurement (a site speeds up only what it wraps; `recv`/`send`
+//! on the serialized stage sit outside the canonical guard):
+//!   work (default) — guard wraps only the 400µs of work.
+//!   wide           — guard widened over recv + work + send, the whole
+//!                    serialized per-item path.
+//!   occupancy      — no experiments; times each segment of reserve's loop
+//!                    at baseline and reports the unguarded per-item
+//!                    overhead δ plus the impact ceiling it implies.
+//!
+//! Result (24-core run, 2026-07-13, job f9305cbb): δ measured 0.3µs/item —
+//! 0.1% of the serialized path — and `wide` does not move the @50% cell
+//! (+83.5/+86.3 vs work's +81.5/+86.7). This demo's +84-vs-+100 @50%
+//! shortfall is therefore NOT unguarded stage time; it is controller-side:
+//! injected delay reaches ~327ms of the ideal 350ms over the 700ms window,
+//! plus a ~3% real-throughput dip while experiments run. Contrast urus's
+//! causal_bench, where the same arithmetic identified a real ~70µs/request
+//! unguarded remainder (recv/reply outside the store guard). Sites measure
+//! what they wrap — and the occupancy probe tells you which case you're in.
+//!
+//! Prints a summary, writes `profile.coz` (Coz plot-compatible), and — given
+//! enough cores for the pipeline to actually run in parallel — checks the
+//! expected separation and exits nonzero if it doesn't hold, so a CI box can
+//! run this as a smoke test.
+
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
+use std::time::{Duration, Instant};
+
+/// LCG-mix `iters` times in dependent sequence (unvectorizable, un-elidable),
+/// staying preemptible — and causal-sampleable/delayable — via `check!()`.
+fn work_iters(iters: u64) {
+    let mut acc = 0x2545_f491_4f6c_dd1du64;
+    let mut i = 0u64;
+    while i < iters {
+        let chunk_end = (i + 256).min(iters);
+        while i < chunk_end {
+            acc = acc.wrapping_mul(6364136223846793005).wrapping_add(i);
+            i += 1;
+        }
+        std::hint::black_box(acc);
+        smarm::check!();
+    }
+}
+
+/// Measure how many `work_iters` iterations fit in a microsecond on this
+/// machine, so stage costs below are meaningful in time while staying
+/// work-shaped.
+fn calibrate_iters_per_us() -> u64 {
+    let n = 8_000_000u64;
+    let t = Instant::now();
+    work_iters(n);
+    (n / (t.elapsed().as_micros().max(1) as u64)).max(1)
+}
+
+/// Guard placement for the `reserve` stage — see module doc.
+#[derive(Clone, Copy, PartialEq)]
+enum Mode {
+    Work,
+    Wide,
+    Occupancy,
+}
+
+fn main() {
+    let mode = match std::env::var("SMARM_CAUSAL_MODE").as_deref() {
+        Err(_) | Ok("") | Ok("work") => Mode::Work,
+        Ok("wide") => Mode::Wide,
+        Ok("occupancy") => Mode::Occupancy,
+        Ok(other) => {
+            eprintln!("unknown SMARM_CAUSAL_MODE {other:?} (work|wide|occupancy)");
+            std::process::exit(2);
+        }
+    };
+    println!(
+        "mode: {}",
+        match mode {
+            Mode::Work => "work",
+            Mode::Wide => "wide",
+            Mode::Occupancy => "occupancy",
+        }
+    );
+    let per_us = calibrate_iters_per_us();
+    println!("calibration: {per_us} work iters/µs");
+    let work_us = move |us: u64| work_iters(us * per_us);
+
+    let mut failures: Vec<String> = Vec::new();
+
+    smarm::init(smarm::Config::default()).run(move || {
+        let stop = Arc::new(AtomicBool::new(false));
+
+        let (tx_ab, rx_ab) = smarm::channel::<u64>();
+        let (tx_bc, rx_bc) = smarm::channel::<u64>();
+
+        // Producer: hot serialization, but upstream of the bottleneck.
+        let stop_p = stop.clone();
+        let producer = smarm::spawn(move || {
+            let mut i = 0u64;
+            while !stop_p.load(Ordering::Relaxed) {
+                {
+                    let _g = smarm::causal_site!("serialize");
+                    work_us(200);
+                }
+                if tx_ab.send(i).is_err() {
+                    break;
+                }
+                i += 1;
+            }
+            // tx_ab drops here; downstream drains and exits.
+        });
+
+        // Reserve: the true bottleneck (~400µs of work per item).
+        let reserve = smarm::spawn(move || match mode {
+            Mode::Work => {
+                while let Ok(item) = rx_ab.recv() {
+                    {
+                        let _g = smarm::causal_site!("reserve");
+                        work_us(400);
+                    }
+                    if tx_bc.send(item).is_err() {
+                        break;
+                    }
+                }
+            }
+            // Whole serialized per-item path under the guard: a virtual
+            // speedup now also compresses recv/send, so the @50% cell should
+            // recover the theoretical 2× that `work` mode's placement caps.
+            Mode::Wide => loop {
+                let _g = smarm::causal_site!("reserve");
+                let Ok(item) = rx_ab.recv() else { break };
+                work_us(400);
+                if tx_bc.send(item).is_err() {
+                    break;
+                }
+            },
+            // Time each segment at baseline; the recv+send remainder δ is
+            // the serialized time a `work`-placed guard cannot speed up.
+            Mode::Occupancy => {
+                let (mut recv_ns, mut work_ns, mut send_ns, mut n) = (0u64, 0u64, 0u64, 0u64);
+                loop {
+                    let t0 = Instant::now();
+                    let Ok(item) = rx_ab.recv() else { break };
+                    let t1 = Instant::now();
+                    {
+                        let _g = smarm::causal_site!("reserve");
+                        work_us(400);
+                    }
+                    let t2 = Instant::now();
+                    if tx_bc.send(item).is_err() {
+                        break;
+                    }
+                    recv_ns += (t1 - t0).as_nanos() as u64;
+                    work_ns += (t2 - t1).as_nanos() as u64;
+                    send_ns += t2.elapsed().as_nanos() as u64;
+                    n += 1;
+                }
+                let items = n.max(1) as f64;
+                let (r, w, s) = (
+                    recv_ns as f64 / items / 1e3,
+                    work_ns as f64 / items / 1e3,
+                    send_ns as f64 / items / 1e3,
+                );
+                let delta = r + s;
+                let total = w + delta;
+                println!("occupancy: {n} items; per item recv {r:.1}µs + work(guarded) {w:.1}µs + send {s:.1}µs");
+                println!(
+                    "occupancy: unguarded δ = {delta:.1}µs/item = {:.1}% of the serialized path",
+                    100.0 * delta / total
+                );
+                for pct in [25u32, 50] {
+                    let f = 1.0 - f64::from(pct) / 100.0;
+                    println!(
+                        "occupancy: predicted reserve impact @{pct}% -> {:+.1}%  (ceiling if δ were guarded: {:+.1}%)",
+                        100.0 * (total / (f * w + delta) - 1.0),
+                        100.0 * (1.0 / f - 1.0)
+                    );
+                }
+            }
+        });
+
+        // Notify: light tail stage; marks the unit of useful work.
+        let notify = smarm::spawn(move || {
+            while rx_bc.recv().is_ok() {
+                {
+                    let _g = smarm::causal_site!("notify");
+                    work_us(50);
+                }
+                smarm::progress!("orders-processed");
+            }
+        });
+
+        // Background: hottest code in the process, zero throughput relevance.
+        let stop_bg = stop.clone();
+        let background = smarm::spawn(move || {
+            while !stop_bg.load(Ordering::Relaxed) {
+                let _g = smarm::causal_site!("background-compaction");
+                work_us(500);
+            }
+        });
+
+        // Warm up so queues reach steady state before measuring.
+        smarm::sleep(Duration::from_millis(300));
+
+        if mode == Mode::Occupancy {
+            // No experiments: hold steady state for a window, then drain and
+            // let the reserve actor print its segment report.
+            smarm::sleep(Duration::from_millis(1500));
+            stop.store(true, Ordering::Relaxed);
+            producer.join().unwrap();
+            reserve.join().unwrap();
+            notify.join().unwrap();
+            background.join().unwrap();
+            return;
+        }
+
+        let results = smarm::causal::run_experiments(&smarm::causal::ExperimentPlan {
+            speedups_pct: vec![0, 25, 50],
+            experiment: Duration::from_millis(700),
+            cooldown: Duration::from_millis(150),
+        });
+
+        stop.store(true, Ordering::Relaxed);
+        producer.join().unwrap();
+        reserve.join().unwrap();
+        notify.join().unwrap();
+        background.join().unwrap();
+
+        print!("{}", smarm::causal::render_summary(&results));
+        // RFC 007 deficit hunt: SMARM_CAUSAL_AUDIT=1 appends the per-cell
+        // ledger audit (injected/absorbed/forgiven + drop and discard
+        // buckets) without touching the pinned summary format.
+        if std::env::var_os("SMARM_CAUSAL_AUDIT").is_some() {
+            print!("{}", smarm::causal::render_ledger_audit(&results));
+        }
+        let coz = smarm::causal::render_coz(&results);
+        match std::fs::write("profile.coz", coz) {
+            Ok(()) => println!("\nwrote profile.coz"),
+            Err(e) => eprintln!("\nfailed to write profile.coz: {e}"),
+        }
+
+        // Verdict. The separation only exists when the four pipeline actors
+        // actually run in parallel; on a small box, report and skip.
+        let cores = std::thread::available_parallelism().map(|n| n.get()).unwrap_or(1);
+        if cores < 4 {
+            println!("verdict: SKIPPED ({cores} cores; separation needs the stages in parallel)");
+            return;
+        }
+        let impact = |site: &str| {
+            smarm::causal::impact_pct(&results, site, 25, "orders-processed")
+        };
+        let mut expect = |site: &str, ok: &dyn Fn(f64) -> bool, want: &str| match impact(site) {
+            Some(p) => {
+                let verdict = if ok(p) { "ok" } else { "FAIL" };
+                println!("verdict: {site} @25% -> {p:+.1}% (want {want}) {verdict}");
+                if !ok(p) {
+                    failures.push(format!("{site}: {p:+.1}% (want {want})"));
+                }
+            }
+            None => {
+                println!("verdict: {site} @25% -> missing cell FAIL");
+                failures.push(format!("{site}: missing cell"));
+            }
+        };
+        expect("reserve", &|p| p > 15.0, "> +15%");
+        expect("serialize", &|p| p < 10.0, "< +10%");
+        expect("background-compaction", &|p| p < 10.0, "< +10%");
+
+        if failures.is_empty() {
+            println!("verdict: PASS — causal separation holds");
+        } else {
+            println!("verdict: FAIL — {}", failures.join("; "));
+            std::process::exit(1);
+        }
+    });
+}

examples/causal_probe.rs

@@ -0,0 +1,145 @@
+//! Diagnostic probe for RFC 007 on a target box. Measures, in order:
+//!  1. TSC frequency against `Instant` (the crate assumes 3 GHz).
+//!  2. TSC sanity under actor migration: distribution of wall time actually
+//!     spent in `burn_us(400)` across many runs — a bimodal/short tail means
+//!     cross-core TSC offsets are cutting burns short.
+//!  3. Pipeline stage rates with no experiment running (who is the real
+//!     bottleneck?).
+//!  4. The same rates during a 50% experiment on `background-compaction`
+//!     (a correct implementation must slow every stage; an off-critical-path
+//!     target must reduce end-to-end throughput proportionally).
+//!
+//! Run: cargo run --release --example causal_probe --features smarm-causal
+
+use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
+use std::sync::Arc;
+use std::time::{Duration, Instant};
+
+fn burn_us(us: u64) {
+    let cycles = us * 3_000;
+    let start = smarm::preempt::rdtsc();
+    while smarm::preempt::rdtsc().saturating_sub(start) < cycles {
+        smarm::check!();
+    }
+}
+
+fn main() {
+    // 1. TSC calibration (plain OS thread, before the runtime starts).
+    let c0 = smarm::preempt::rdtsc();
+    let t0 = Instant::now();
+    std::thread::sleep(Duration::from_millis(200));
+    let hz = (smarm::preempt::rdtsc() - c0) as f64 / t0.elapsed().as_secs_f64();
+    println!("tsc_hz: {:.3e} (crate assumes 3.0e9)", hz);
+
+    smarm::init(smarm::Config::default()).run(move || {
+        // 2. burn_us(400) wall-time distribution inside a migrating actor.
+        let h = smarm::spawn(|| {
+            let mut samples: Vec<u64> = (0..500)
+                .map(|_| {
+                    let t = Instant::now();
+                    burn_us(400);
+                    t.elapsed().as_micros() as u64
+                })
+                .collect();
+            samples.sort_unstable();
+            println!(
+                "burn_us(400) wall us: min {} p10 {} p50 {} p90 {} max {}",
+                samples[0], samples[50], samples[250], samples[450], samples[499]
+            );
+        });
+        h.join().unwrap();
+
+        // 3+4. Pipeline with per-stage counters.
+        let stop = Arc::new(AtomicBool::new(false));
+        let produced = Arc::new(AtomicU64::new(0));
+        let reserved = Arc::new(AtomicU64::new(0));
+        let notified = Arc::new(AtomicU64::new(0));
+
+        let (tx_ab, rx_ab) = smarm::channel::<u64>();
+        let (tx_bc, rx_bc) = smarm::channel::<u64>();
+
+        let stop_p = stop.clone();
+        let produced2 = produced.clone();
+        let producer = smarm::spawn(move || {
+            let mut i = 0u64;
+            while !stop_p.load(Ordering::Relaxed) {
+                {
+                    let _g = smarm::causal_site!("serialize");
+                    burn_us(200);
+                }
+                if tx_ab.send(i).is_err() {
+                    break;
+                }
+                produced2.fetch_add(1, Ordering::Relaxed);
+                i += 1;
+            }
+        });
+
+        let reserved2 = reserved.clone();
+        let reserve = smarm::spawn(move || {
+            while let Ok(item) = rx_ab.recv() {
+                {
+                    let _g = smarm::causal_site!("reserve");
+                    burn_us(400);
+                }
+                reserved2.fetch_add(1, Ordering::Relaxed);
+                if tx_bc.send(item).is_err() {
+                    break;
+                }
+            }
+        });
+
+        let notified2 = notified.clone();
+        let notify = smarm::spawn(move || {
+            while rx_bc.recv().is_ok() {
+                {
+                    let _g = smarm::causal_site!("notify");
+                    burn_us(50);
+                }
+                notified2.fetch_add(1, Ordering::Relaxed);
+                smarm::progress!("orders-processed");
+            }
+        });
+
+        let stop_bg = stop.clone();
+        let background = smarm::spawn(move || {
+            while !stop_bg.load(Ordering::Relaxed) {
+                let _g = smarm::causal_site!("background-compaction");
+                burn_us(500);
+            }
+        });
+
+        smarm::sleep(Duration::from_millis(300));
+
+        let window = |label: &str| {
+            let (p0, r0, n0) = (
+                produced.load(Ordering::Relaxed),
+                reserved.load(Ordering::Relaxed),
+                notified.load(Ordering::Relaxed),
+            );
+            let d0 = smarm::causal::global_delay_cycles();
+            let t = Instant::now();
+            smarm::sleep(Duration::from_millis(700));
+            let secs = t.elapsed().as_secs_f64();
+            println!(
+                "{label}: produced {:.0}/s reserved {:.0}/s notified {:.0}/s injected {:.0}ms(assumed-3GHz)",
+                (produced.load(Ordering::Relaxed) - p0) as f64 / secs,
+                (reserved.load(Ordering::Relaxed) - r0) as f64 / secs,
+                (notified.load(Ordering::Relaxed) - n0) as f64 / secs,
+                (smarm::causal::global_delay_cycles() - d0) as f64 / 3.0e9 * 1e3,
+            );
+        };
+
+        window("no-experiment ");
+        smarm::causal::begin_experiment_for_test("background-compaction", 50);
+        window("bg-comp @ 50% ");
+        smarm::causal::end_experiment_for_test();
+        window("post-experiment");
+
+        stop.store(true, Ordering::Relaxed);
+        producer.join().unwrap();
+        reserve.join().unwrap();
+        notify.join().unwrap();
+        background.join().unwrap();
+    });
+}

examples/gen_statem_expanded.rs

@@ -31,12 +31,12 @@
 //! Default build is clean and runs. A BREAK-CASE MENU at the bottom documents
 //! how to make each of the four guarantees fire.
 //!
-//! Run: `cargo run --example gen_statem_fused`
+//! Run: `cargo run --example gen_statem_expanded`
 
 #![deny(dead_code, unreachable_patterns)]
 
 use smarm::run;
-use smarm::gen_statem::{spawn, Cx, Machine, Reply, Resolution, GenStatemRef};
+use smarm::gen_statem::{spawn, Cx, Machine, Reply, Resolution, Step, GenStatemRef};
 
 // === user types ============================================================
 
@@ -50,6 +50,7 @@ enum Door {
 struct Data {
     enters: u32, // total state entries (incl. initial)
     pushes: u32, // times a push closed the door
+    knocks: u32, // knocks answered (a Locked knock is postponed, then counted)
 }
 
 enum Cast {
@@ -57,17 +58,24 @@ enum Cast {
     Pull,
     Lock,
     Unlock(u32), // carries a key
+    Knock,       // counted when the door is reachable; postponed while Locked
 }
 
 enum Call {
     GetState(Reply<Door>),
     GetEnters(Reply<u32>),
     GetPushes(Reply<u32>),
+    GetKnocks(Reply<u32>),
 }
 
 enum Ev {
     Cast(Cast),
     Call(Call),
+    // The runtime's internal events. `Info` is out-of-band (here unused, so
+    // `()`); `StateTimeout` / `Timeout` are timer fires the loop feeds back in.
+    Info(()),
+    StateTimeout,
+    Timeout(&'static str),
 }
 
 const CODE: u32 = 1234;
@@ -115,34 +123,62 @@ impl DoorSm {
     fn start(init: Door) -> GenStatemRef<DoorSm> {
         spawn(DoorSm {
             state: init,
-            data: Data { enters: 0, pushes: 0 },
+            data: Data { enters: 0, pushes: 0, knocks: 0 },
         })
     }
 
-    fn enter(&mut self, _cx: &mut Cx<Ev>) {
+    fn enter(&mut self, cx: &mut Cx<Ev>) {
         self.data.enters += 1;
+        // A state-timeout: an Open door auto-closes after a quiet window. The
+        // loop auto-resets it on any transition, so it fires only if the door is
+        // still Open when it elapses.
+        if self.state == Door::Open {
+            cx.state_timeout(std::time::Duration::from_millis(5));
+        }
     }
 }
 
 impl Machine for DoorSm {
     type Ev = Ev;
 
+    fn state_timeout_ev() -> Ev {
+        Ev::StateTimeout
+    }
+
+    fn timeout_ev(name: &'static str) -> Ev {
+        Ev::Timeout(name)
+    }
+
     fn on_start(&mut self, cx: &mut Cx<Ev>) {
         self.enter(cx);
     }
 
-    fn handle(&mut self, ev: Ev, cx: &mut Cx<Ev>) {
+    fn handle(&mut self, ev: Ev, cx: &mut Cx<Ev>) -> Step<Ev> {
         let prev = self.state;
 
-        // ---- the transition table -----------------------------------------
-        // This match is total over (Door, Ev). Read it as the declared graph:
-        // each `=> To(x)` is an edge, each `=> Unhandled` an explicit refusal.
+        // ---- phase 1: postpone routing (borrow-only) ----------------------
+        // A deferred event is handed back untouched for the loop's postpone
+        // queue; no handler code runs on it. Here: a knock at a locked door.
+        // (The macro emits this as a `match (state, &ev)` yielding a bool; the
+        // hand-written form can just test directly.)
+        if let (Door::Locked, Ev::Cast(Cast::Knock)) = (prev, &ev) {
+            return Step::Postponed(ev);
+        }
+
+        // ---- phase 2: the consuming transition table ----------------------
+        // Total over (Door, Ev). Read it as the declared graph: each `=> To(x)`
+        // is an edge, each `=> Unhandled` an explicit refusal. The postponed
+        // pair above reappears as `unreachable!` so the match stays total.
         let res: Resolution<Door> = match (self.state, ev) {
             // --- Open -------------------------------------------------------
             (Door::Open, Ev::Cast(Cast::Push)) => {
                 on_push(&mut self.data);
                 Resolution::To(Door::Closed)
             }
+            (Door::Open, Ev::Cast(Cast::Knock)) => {
+                self.data.knocks += 1;
+                Resolution::To(prev)
+            }
             (Door::Open, Ev::Cast(Cast::Pull | Cast::Lock | Cast::Unlock(_))) => {
                 Resolution::Unhandled
             }
@@ -150,12 +186,20 @@ impl Machine for DoorSm {
             // --- Closed -----------------------------------------------------
             (Door::Closed, Ev::Cast(Cast::Pull)) => Resolution::To(Door::Open),
             (Door::Closed, Ev::Cast(Cast::Lock)) => Resolution::To(Door::Locked),
+            (Door::Closed, Ev::Cast(Cast::Knock)) => {
+                self.data.knocks += 1;
+                Resolution::To(prev)
+            }
             (Door::Closed, Ev::Cast(Cast::Push | Cast::Unlock(_))) => Resolution::Unhandled,
 
             // --- Locked (branching row: handler picks within UnlockOutcome) -
             (Door::Locked, Ev::Cast(Cast::Unlock(key))) => {
                 Resolution::To(on_unlock(key).into())
             }
+            // Routed out in phase 1; listed only to keep this match total.
+            (Door::Locked, Ev::Cast(Cast::Knock)) => {
+                unreachable!("postponed event is replayed, not dispatched here")
+            }
             (Door::Locked, Ev::Cast(Cast::Push | Cast::Pull | Cast::Lock)) => {
                 Resolution::Unhandled
             }
@@ -173,17 +217,38 @@ impl Machine for DoorSm {
                 r.reply(self.data.pushes);
                 Resolution::To(prev)
             }
+            (_, Ev::Call(Call::GetKnocks(r))) => {
+                r.reply(self.data.knocks);
+                Resolution::To(prev)
+            }
+
+            // --- timeouts: an Open door auto-closes; others have none armed --
+            (Door::Open, Ev::StateTimeout) => Resolution::To(Door::Closed),
+            (_, Ev::StateTimeout) => Resolution::Unhandled,
+            (_, Ev::Timeout(name)) => {
+                // No named timeout is armed in this run; a real handler would
+                // dispatch on `name`. Acknowledge it to exercise the field.
+                let _ = name;
+                Resolution::Unhandled
+            }
+
+            // --- out-of-band info: silent drop (the gen_server default) ------
+            (_, Ev::Info(_)) => Resolution::Unhandled,
         };
 
         // ---- apply the resolution -----------------------------------------
         match res {
-            Resolution::To(s) if s == prev => {} // stay: no enter
+            Resolution::To(s) if s == prev => Step::Stayed, // stay: no enter
             Resolution::To(s) => {
                 self.state = s; // sole writer of the state cell
+                cx.__reset_state_timeout(); // auto-reset across transitions
                 self.enter(cx);
+                Step::Transitioned
+            }
+            Resolution::Unhandled => {
+                cx.on_unhandled();
+                Step::Stayed
             }
-            Resolution::Postpone => unreachable!("postpone is not generated yet"),
-            Resolution::Unhandled => cx.on_unhandled(),
         }
     }
 }
@@ -193,20 +258,29 @@ fn main() {
         let door = DoorSm::start(Door::Closed);
 
         door.send(Ev::Cast(Cast::Lock)).unwrap(); // Closed -> Locked
+        door.send(Ev::Cast(Cast::Knock)).unwrap(); // Locked: postponed (not yet counted)
         door.send(Ev::Cast(Cast::Push)).unwrap(); // Locked: Push invalid -> Unhandled
-        door.send(Ev::Cast(Cast::Unlock(0))).unwrap(); // Locked: wrong key -> stay
-        door.send(Ev::Cast(Cast::Unlock(CODE))).unwrap(); // Locked -> Closed
-        door.send(Ev::Cast(Cast::Pull)).unwrap(); // Closed -> Open
-        door.send(Ev::Cast(Cast::Push)).unwrap(); // Open  -> Closed (pushes=1)
+        door.send(Ev::Cast(Cast::Unlock(0))).unwrap(); // Locked: wrong key -> stay (knock still deferred)
+        door.send(Ev::Cast(Cast::Unlock(CODE))).unwrap(); // Locked -> Closed; deferred Knock replays here
+        door.send(Ev::Cast(Cast::Push)).unwrap(); // Closed: Push invalid -> Unhandled
+        door.send(Ev::Cast(Cast::Pull)).unwrap(); // Closed -> Open (arms 5ms auto-close)
+        door.send(Ev::Info(())).unwrap(); // out-of-band: silently dropped
+
+        // Wait past the auto-close window: the state-timeout fires and the door
+        // closes itself, with no further input. (`smarm::sleep` parks the actor
+        // without blocking a worker thread, so the timer wheel keeps turning.)
+        smarm::sleep(std::time::Duration::from_millis(40));
 
         let st = door.call(|r| Ev::Call(Call::GetState(r))).unwrap();
         let enters = door.call(|r| Ev::Call(Call::GetEnters(r))).unwrap();
         let pushes = door.call(|r| Ev::Call(Call::GetPushes(r))).unwrap();
+        let knocks = door.call(|r| Ev::Call(Call::GetKnocks(r))).unwrap();
 
-        println!("state={st:?} enters={enters} pushes={pushes}");
-        assert_eq!(st, Door::Closed);
+        println!("state={st:?} enters={enters} pushes={pushes} knocks={knocks}");
+        assert_eq!(st, Door::Closed); // auto-closed by the state-timeout
         assert_eq!(enters, 5); // Closed(start) + Locked + Closed + Open + Closed
-        assert_eq!(pushes, 1);
+        assert_eq!(pushes, 0); // no push ever closed it this run
+        assert_eq!(knocks, 1); // the locked-door knock, replayed once unlocked
         println!("ok");
     });
 }

examples/gen_statem_macro.rs

@@ -1,4 +1,4 @@
-//! The **same** machine as `examples/gen_statem_fused.rs`, written through the
+//! The **same** machine as `examples/gen_statem_expanded.rs`, written through the
 //! `gen_statem!` macro. Diff this file against that one to see exactly what the
 //! macro buys: every `// ===` section there that was boilerplate (the `Ev`
 //! enum, the `DoorSm` struct, `start`, the whole `Machine` impl, the `enter`
@@ -21,7 +21,7 @@ use smarm::gen_statem;
 use smarm::run;
 use smarm::gen_statem::Reply;
 
-// === user types (identical to gen_statem_fused.rs) =========================
+// === user types (identical to gen_statem_expanded.rs) =========================
 
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]
 enum Door {
@@ -33,6 +33,7 @@ enum Door {
 struct Data {
     enters: u32, // total state entries (incl. initial)
     pushes: u32, // times a push closed the door
+    knocks: u32, // knocks answered (a Locked knock is postponed, then counted)
 }
 
 enum Cast {
@@ -40,12 +41,14 @@ enum Cast {
     Pull,
     Lock,
     Unlock(u32), // carries a key
+    Knock,       // counted when the door is reachable; postponed while Locked
 }
 
 enum Call {
     GetState(Reply<Door>),
     GetEnters(Reply<u32>),
     GetPushes(Reply<u32>),
+    GetKnocks(Reply<u32>),
 }
 
 const CODE: u32 = 1234;
@@ -87,7 +90,7 @@ fn on_unlock(key: u32) -> UnlockOutcome {
 
 gen_statem! {
     machine: DoorSm { state: Door, data: Data };
-    event:   Ev { cast: Cast, call: Call };
+    event:   Ev { cast: Cast, call: Call, info: () };
 
     // You name the bindings the bodies use; the macro can't lend you its own
     // `self`/`cx` across macro hygiene. `data` = &mut Data, `prev` = current
@@ -100,15 +103,20 @@ gen_statem! {
 
     on Door::Open => {
         cast Cast::Push                            => { on_push(data); Door::Closed },
+        cast Cast::Knock                           => { data.knocks += 1; prev },
         cast Cast::Pull | Cast::Lock | Cast::Unlock(_) => unhandled,
     }
     on Door::Closed => {
         cast Cast::Pull                            => Door::Open,
         cast Cast::Lock                            => Door::Locked,
+        cast Cast::Knock                           => { data.knocks += 1; prev },
         cast Cast::Push | Cast::Unlock(_)          => unhandled,
     }
     on Door::Locked => {
         cast Cast::Unlock(key)                     => on_unlock(key), // branch -> UnlockOutcome
+        // A knock at a locked door waits: defer it until the door is reachable,
+        // where the replay counts it.
+        cast Cast::Knock                           => postpone,
         cast Cast::Push | Cast::Pull | Cast::Lock  => unhandled,
     }
 
@@ -117,35 +125,43 @@ gen_statem! {
         call Call::GetState(r)  => { r.reply(prev); prev },
         call Call::GetEnters(r) => { r.reply(data.enters); prev },
         call Call::GetPushes(r) => { r.reply(data.pushes); prev },
+        call Call::GetKnocks(r) => { r.reply(data.knocks); prev },
+        // This machine arms no timeouts, so refuse them everywhere. (Info has a
+        // built-in silent-drop default, so it needs no row.)
+        state_timeout          => unhandled,
+        timeout _              => unhandled,
     }
 }
 
 fn main() {
     run(|| {
-        let door = DoorSm::start(Door::Closed, Data { enters: 0, pushes: 0 });
+        let door = DoorSm::start(Door::Closed, Data { enters: 0, pushes: 0, knocks: 0 });
 
         door.send(Ev::Cast(Cast::Lock)).unwrap(); // Closed -> Locked
+        door.send(Ev::Cast(Cast::Knock)).unwrap(); // Locked: postponed (not yet counted)
         door.send(Ev::Cast(Cast::Push)).unwrap(); // Locked: Push invalid -> Unhandled
-        door.send(Ev::Cast(Cast::Unlock(0))).unwrap(); // Locked: wrong key -> stay
-        door.send(Ev::Cast(Cast::Unlock(CODE))).unwrap(); // Locked -> Closed
+        door.send(Ev::Cast(Cast::Unlock(0))).unwrap(); // Locked: wrong key -> stay (knock still deferred)
+        door.send(Ev::Cast(Cast::Unlock(CODE))).unwrap(); // Locked -> Closed; deferred Knock replays here
         door.send(Ev::Cast(Cast::Pull)).unwrap(); // Closed -> Open
         door.send(Ev::Cast(Cast::Push)).unwrap(); // Open  -> Closed (pushes=1)
 
         let st = door.call(|r| Ev::Call(Call::GetState(r))).unwrap();
         let enters = door.call(|r| Ev::Call(Call::GetEnters(r))).unwrap();
         let pushes = door.call(|r| Ev::Call(Call::GetPushes(r))).unwrap();
+        let knocks = door.call(|r| Ev::Call(Call::GetKnocks(r))).unwrap();
 
-        println!("state={st:?} enters={enters} pushes={pushes}");
+        println!("state={st:?} enters={enters} pushes={pushes} knocks={knocks}");
         assert_eq!(st, Door::Closed);
         assert_eq!(enters, 5); // Closed(start) + Locked + Closed + Open + Closed
         assert_eq!(pushes, 1);
+        assert_eq!(knocks, 1); // the locked-door knock, replayed once unlocked
         println!("ok");
     });
 }
 
 // ===========================================================================
 // BREAK-CASE MENU — the four guarantees, through the macro. Each fires exactly
-// as it does in the hand-written gen_statem_fused.rs.
+// as it does in the hand-written gen_statem_expanded.rs.
 //
 // 1. ORPHAN HANDLER (dead_code -> error):
 //      add `fn on_slam(_d: &mut Data) {}` and don't reference it.

src/actor.rs

@@ -93,8 +93,10 @@ pub fn take_last_outcome() -> Option<Outcome> {
 /// unwinding to cross the boundary, but `catch_unwind` here means unwinding
 /// never actually does.
 pub extern "C-unwind" fn trampoline() {
-    let b = CURRENT_ACTOR_BOX.with(|c| c.borrow_mut().take())
-        .expect("trampoline entered without a closure set");
+    let b = match CURRENT_ACTOR_BOX.with(|c| c.borrow_mut().take()) {
+        Some(b) => b,
+        None => panic!("smarm: trampoline entered without a closure set (core corrupt)"),
+    };
 
     let outcome = match panic::catch_unwind(panic::AssertUnwindSafe(b)) {
         Ok(())       => Outcome::Exit,

src/causal.rs

@@ -0,0 +1,959 @@
+//! Native causal profiling (RFC 007). Enabled by `--features smarm-causal`;
+//! zero cost without it (same discipline as `smarm-trace`).
+//!
+//! The Coz algorithm, transposed onto actors: to estimate what speeding up
+//! code site S by p% would do to throughput, we instead *slow everything
+//! else down* by p% of the time spent in S, and watch the progress-point
+//! rates respond. Where Coz must inject real `usleep`s into OS threads from
+//! the outside, smarm owns every clock that matters:
+//!
+//! - Sampling and delay injection happen at `maybe_preempt`'s amortised
+//!   cadence — an existing, safe hook (never inside a prep-to-park region).
+//! - Injected delay is subtracted from the actor's timeslice
+//!   (`preempt::extend_timeslice`), so experiments don't perturb scheduling.
+//! - Delay bookkeeping is *actor*-granular: each `Slot` carries an absorbed-
+//!   delay ledger, compared against a global ledger. Parked actors absorb
+//!   accrued delay for free on resume (Coz's blocked-thread rule) — waiting
+//!   is never penalised.
+//!
+//! v1 scope (per RFC discussion): explicit scoped sites (`causal_site!`)
+//! rather than PC sampling (jar Q1 stays open); throughput progress points
+//! only; timer-heap deadlines are *not* shifted (documented gap — long
+//! experiments can make real-time timeouts fire early in virtual terms);
+//! multi-scheduler coherence is best-effort via global atomics.
+//!
+//! Usage:
+//! ```ignore
+//! let _g = smarm::causal_site!("inventory-reserve");   // in suspect code
+//! smarm::progress!("orders-processed");                // per unit of work
+//! let results = smarm::causal::run_experiments(&Default::default());
+//! print!("{}", smarm::causal::render_summary(&results));
+//! std::fs::write("profile.coz", smarm::causal::render_coz(&results))?;
+//! ```
+
+#[cfg(feature = "smarm-causal")]
+mod inner {
+    use crate::preempt;
+    use std::cell::Cell;
+    use std::sync::atomic::{AtomicU64, Ordering};
+    use std::sync::{Mutex, OnceLock};
+    use std::time::{Duration, Instant};
+
+    // -----------------------------------------------------------------------
+    // Global state
+    // -----------------------------------------------------------------------
+
+    /// Active experiment, packed `(site_id << 32) | speedup_pct`. 0 = idle.
+    /// A single word so the hot path reads one atomic; experiments are global
+    /// across scheduler threads (jar Q7, v1: plain Relaxed atomics).
+    static EXPERIMENT: AtomicU64 = AtomicU64::new(0);
+
+    /// Monotone experiment-window counter, bumped by every `begin()`. Lets
+    /// the offcpu-gap stash (RFC 007) tell apart two windows with an
+    /// identical site+pct word — live in the attrib probe's 50,50 schedule
+    /// — so a gap straddling `end()`/`begin()` never counts a cooldown.
+    static EXPERIMENT_EPOCH: AtomicU64 = AtomicU64::new(0);
+
+    /// Global virtual-delay ledger, in TSC cycles: the total delay every
+    /// actor *should* have experienced since startup. Grows while a sample
+    /// lands in the experiment's target site; each actor's `Slot` ledger
+    /// chases it by spin-absorbing at preemption checks.
+    static GLOBAL_DELAY: AtomicU64 = AtomicU64::new(0);
+
+    /// Registered site names; site id = index + 1 (0 = "no site").
+    static SITES: OnceLock<Mutex<Vec<&'static str>>> = OnceLock::new();
+
+    /// Registered progress points (leaked for `'static`, like trace's drain
+    /// state — the set is small and lives for the process).
+    static PROGRESS: OnceLock<Mutex<Vec<&'static ProgressPoint>>> = OnceLock::new();
+
+    thread_local! {
+        /// TSC at this thread's previous causal check, the sample "period"
+        /// denominator. Re-armed on every actor resume so scheduler time and
+        /// a previous actor's tail never count toward a sample. 0 = unarmed.
+        static LAST_SAMPLE_TSC: Cell<u64> = const { Cell::new(0) };
+    }
+
+    /// Guard against TSC weirdness (migration between unsynced sockets,
+    /// virtualisation steps): a single sample interval larger than this is
+    /// discarded rather than believed. ~33ms at 3 GHz — far beyond any real
+    /// gap between preemption checks inside a slice.
+    const MAX_SAMPLE_CYCLES: u64 = 100_000_000;
+
+    /// Cap on delay spun in one visit, so one check can never wedge an actor
+    /// for a human-visible pause; the remainder is absorbed on later visits.
+    /// ~3ms at 3 GHz.
+    const MAX_SPIN_PER_VISIT: u64 = 10_000_000;
+
+    // -----------------------------------------------------------------------
+    // Ledger audit (RFC 007 deficit hunt): where injected delay is born,
+    // paid, and forgiven — and where would-be attribution is silently lost
+    // (deschedule tails, clamp discards). Monotone Relaxed totals, read via
+    // `ledger_counters()`; `run_experiments` windows them into
+    // `ExperimentResult`. Measure-only: nothing here changes injection or
+    // absorption behaviour.
+    // -----------------------------------------------------------------------
+
+    /// Cycles bystanders actually spun to pay down the global ledger.
+    static SPIN_ABSORBED_CYCLES: AtomicU64 = AtomicU64::new(0);
+    /// Cycles waived at wake after a real park (the blocked-thread rule).
+    static PARK_FORGIVEN_CYCLES: AtomicU64 = AtomicU64::new(0);
+    /// Would-be attribution lost when the target-site actor parks mid-site.
+    static DROP_PARK_CYCLES: AtomicU64 = AtomicU64::new(0);
+    static DROP_PARK_N: AtomicU64 = AtomicU64::new(0);
+    /// Same loss at yields (explicit, slice-expiry, or a park that requeued).
+    static DROP_YIELD_CYCLES: AtomicU64 = AtomicU64::new(0);
+    static DROP_YIELD_N: AtomicU64 = AtomicU64::new(0);
+    /// Samples discarded by the TSC-weirdness clamp, in would-be delta terms.
+    static DISCARD_OVERMAX_CYCLES: AtomicU64 = AtomicU64::new(0);
+    static DISCARD_OVERMAX_N: AtomicU64 = AtomicU64::new(0);
+    /// In-site samples dropped because the thread's clock was unarmed.
+    static DISCARD_UNARMED_N: AtomicU64 = AtomicU64::new(0);
+    /// Would-be attribution over runnable off-CPU gaps inside the target
+    /// site (yield-descheduled -> resumed within the same window). Not a
+    /// loss: on-CPU-only attribution is the Coz model — queue-wait is not
+    /// shrunk by speeding the site's code — but counted so the audit books
+    /// close against wall in-site time (the located @50 "deficit").
+    static OFFCPU_IN_SITE_CYCLES: AtomicU64 = AtomicU64::new(0);
+    static OFFCPU_IN_SITE_N: AtomicU64 = AtomicU64::new(0);
+
+    fn sites() -> &'static Mutex<Vec<&'static str>> {
+        SITES.get_or_init(|| Mutex::new(Vec::new()))
+    }
+
+    fn progress_points() -> &'static Mutex<Vec<&'static ProgressPoint>> {
+        PROGRESS.get_or_init(|| Mutex::new(Vec::new()))
+    }
+
+    /// Recover from lock poisoning: all these registries hold plain data that
+    /// is valid at every instruction boundary, so a panicked registrant can't
+    /// leave them torn.
+    fn lock_unpoisoned<T>(m: &Mutex<T>) -> std::sync::MutexGuard<'_, T> {
+        match m.lock() {
+            Ok(g) => g,
+            Err(poisoned) => poisoned.into_inner(),
+        }
+    }
+
+    // -----------------------------------------------------------------------
+    // Sites
+    // -----------------------------------------------------------------------
+
+    /// Register (or look up) a causal site by name; returns its nonzero id.
+    /// Called once per `causal_site!` expansion via a `OnceLock`, so the
+    /// mutex is off every hot path.
+    pub fn site_id(name: &'static str) -> u32 {
+        let mut v = lock_unpoisoned(sites());
+        if let Some(pos) = v.iter().position(|n| *n == name) {
+            return (pos + 1) as u32;
+        }
+        v.push(name);
+        v.len() as u32
+    }
+
+    fn site_name(id: u32) -> Option<String> {
+        if id == 0 {
+            return None;
+        }
+        let v = lock_unpoisoned(sites());
+        v.get((id - 1) as usize).map(|s| (*s).to_string())
+    }
+
+    /// RAII marker: while alive, the current *actor* (not thread — the id
+    /// lives in its `Slot` and survives preemption/migration) is "inside"
+    /// the site. Nesting restores the outer site on drop. Inert outside an
+    /// actor (scheduler/OS-thread stacks).
+    pub struct SiteGuard {
+        /// Slot of the actor that entered, null if entered outside an actor.
+        /// Valid for the guard's whole life: the guard lives on the actor's
+        /// stack, and a slot is never reclaimed while its actor is alive —
+        /// the same argument as `preempt::check_cancelled`.
+        slot: *const crate::runtime::Slot,
+        prev: u32,
+    }
+
+    impl SiteGuard {
+        /// Enter `site` for the on-CPU actor.
+        pub fn enter(site: u32) -> Self {
+            let slot = preempt::current_slot_ptr();
+            if slot.is_null() {
+                return SiteGuard { slot, prev: 0 };
+            }
+            // SAFETY: non-null ⇒ points at the on-CPU actor's slot; see the
+            // field docs for the lifetime argument.
+            let prev = unsafe { (*slot).causal_site() };
+            unsafe { (*slot).set_causal_site(site) };
+            site_transition(slot, prev, site);
+            SiteGuard { slot, prev }
+        }
+    }
+
+    impl Drop for SiteGuard {
+        fn drop(&mut self) {
+            if !self.slot.is_null() {
+                // SAFETY (both): as in `enter` — the actor (and thus its
+                // slot) is alive for as long as this guard is on its stack.
+                let site = unsafe { (*self.slot).causal_site() };
+                unsafe { (*self.slot).set_causal_site(self.prev) };
+                site_transition(self.slot, site, self.prev);
+            }
+        }
+    }
+
+    /// Name of the site the on-CPU actor is currently inside, if any.
+    /// (Introspection/testing; not a hot path.)
+    pub fn current_site_name() -> Option<String> {
+        let slot = preempt::current_slot_ptr();
+        if slot.is_null() {
+            return None;
+        }
+        // SAFETY: on-CPU actor's slot, valid for the whole resume.
+        site_name(unsafe { (*slot).causal_site() })
+    }
+
+    // -----------------------------------------------------------------------
+    // Progress points
+    // -----------------------------------------------------------------------
+
+    /// A named throughput counter. One per distinct name; `progress!` call
+    /// sites sharing a name share the counter.
+    pub struct ProgressPoint {
+        name: &'static str,
+        count: AtomicU64,
+    }
+
+    impl ProgressPoint {
+        /// The hot path: one Relaxed RMW. (Contended across actors by design
+        /// — a progress point is a global rate meter.)
+        #[inline]
+        pub fn bump(&self) {
+            self.count.fetch_add(1, Ordering::Relaxed);
+        }
+    }
+
+    /// Register (or look up) a progress point. Called once per `progress!`
+    /// expansion via a `OnceLock`; the mutex is off the hot path.
+    pub fn register_progress(name: &'static str) -> &'static ProgressPoint {
+        let mut v = lock_unpoisoned(progress_points());
+        if let Some(p) = v.iter().find(|p| p.name == name) {
+            return p;
+        }
+        let p: &'static ProgressPoint = Box::leak(Box::new(ProgressPoint {
+            name,
+            count: AtomicU64::new(0),
+        }));
+        v.push(p);
+        p
+    }
+
+    /// Snapshot of all progress points as `(name, count)`.
+    pub fn progress_snapshot() -> Vec<(String, u64)> {
+        lock_unpoisoned(progress_points())
+            .iter()
+            .map(|p| (p.name.to_string(), p.count.load(Ordering::Relaxed)))
+            .collect()
+    }
+
+    // -----------------------------------------------------------------------
+    // The hot hook: sample + absorb
+    // -----------------------------------------------------------------------
+
+    /// Called from `maybe_preempt` at the amortised timeslice-check cadence,
+    /// under the `PREEMPTION_ENABLED` gate (so never in a prep-to-park or
+    /// no-preempt region — spinning here is as safe as yielding is).
+    ///
+    /// One Relaxed load and out when no experiment is running.
+    #[inline]
+    pub(crate) fn check() {
+        let exp = EXPERIMENT.load(Ordering::Relaxed);
+        if exp == 0 {
+            return;
+        }
+        cold_check(exp);
+    }
+
+    /// The experiment-active path, kept out of the inlined fast path.
+    #[cold]
+    fn cold_check(exp: u64) {
+        let slot = preempt::current_slot_ptr();
+        if slot.is_null() {
+            return;
+        }
+        let now = preempt::rdtsc();
+        let last = LAST_SAMPLE_TSC.with(|c| c.replace(now));
+        let target_site = (exp >> 32) as u32;
+        let pct = exp & 0xffff_ffff;
+
+        // SAFETY (both derefs below): non-null ⇒ the on-CPU actor's slot,
+        // never reclaimed while the actor runs — see `check_cancelled`.
+        let my_site = unsafe { (*slot).causal_site() };
+
+        if my_site == target_site && pct > 0 {
+            // A sample landed in the target site: everyone else must fall
+            // behind by pct% of the sampled interval. Grow the global ledger
+            // and credit ourselves the same amount — the credited gap *is*
+            // the virtual speedup.
+            if last == 0 {
+                // Unarmed clock: no interval to attribute — count the loss.
+                DISCARD_UNARMED_N.fetch_add(1, Ordering::Relaxed);
+                return;
+            }
+            // SAFETY: `slot` is the on-CPU actor's slot (checked non-null
+            // above); see `check_cancelled` for the lifetime argument.
+            unsafe { attribute(slot, now.saturating_sub(last), pct) };
+        } else {
+            // Not the winner: chase the global ledger by spinning off the
+            // difference, then push the slice start forward so injected
+            // delay never counts as compute (the clock correction that Coz
+            // cannot do from outside).
+            let global = GLOBAL_DELAY.load(Ordering::Relaxed);
+            let mine = unsafe { (*slot).causal_delay() };
+            if mine >= global {
+                return;
+            }
+            let spin = (global - mine).min(MAX_SPIN_PER_VISIT);
+            let start = preempt::rdtsc();
+            while preempt::rdtsc().saturating_sub(start) < spin {
+                core::hint::spin_loop();
+            }
+            SPIN_ABSORBED_CYCLES.fetch_add(spin, Ordering::Relaxed);
+            unsafe { (*slot).set_causal_delay(mine.wrapping_add(spin)) };
+            preempt::extend_timeslice(spin);
+            // The spin is not part of the next sample interval either.
+            LAST_SAMPLE_TSC.with(|c| c.set(preempt::rdtsc()));
+        }
+    }
+
+    /// Attribute one target-site sample of `interval` cycles at `pct`%:
+    /// grow the global ledger and credit the sampling actor's own ledger by
+    /// the same amount — the credited gap *is* the virtual speedup. Shared
+    /// by the cold check and the guard-boundary flush. Applies the same
+    /// clamps as sampling always has: zero intervals and clock hiccups are
+    /// discarded, not the run.
+    ///
+    /// SAFETY: `slot` must point at the on-CPU actor's slot (the
+    /// `check_cancelled` lifetime argument).
+    unsafe fn attribute(slot: *const crate::runtime::Slot, interval: u64, pct: u64) {
+        if interval == 0 {
+            return; // now == last: nothing to attribute, nothing lost
+        }
+        if interval > MAX_SAMPLE_CYCLES {
+            // TSC-weirdness clamp: the sample is discarded, not the run.
+            // Count the loss in would-be delta terms so the audit's columns
+            // compare directly against `injected_cycles`.
+            DISCARD_OVERMAX_N.fetch_add(1, Ordering::Relaxed);
+            DISCARD_OVERMAX_CYCLES
+                .fetch_add(interval.saturating_mul(pct) / 100, Ordering::Relaxed);
+            return;
+        }
+        let delta = interval.saturating_mul(pct) / 100;
+        GLOBAL_DELAY.fetch_add(delta, Ordering::Relaxed);
+        let mine = (*slot).causal_delay();
+        (*slot).set_causal_delay(mine.wrapping_add(delta));
+    }
+
+    /// Site-boundary hook, called by `SiteGuard` enter/drop when the
+    /// actor's current site changes from `old` to `new`. Sample-only —
+    /// never spins — so it is safe anywhere, including no-preempt regions
+    /// where `check()` cannot run.
+    ///
+    /// - Leaving the experiment's target site: flush the pending interval.
+    ///   Cold checks only sample when they happen to fire in-site, so the
+    ///   tail between the last check and the guard drop was otherwise
+    ///   discarded on every site entry — measured live at ~22-29µs/entry,
+    ///   ~6-7% of all target time (eff 0.93), which under-reported every
+    ///   impact (+83.5% where theory says +100%).
+    /// - Entering the target site: re-arm the sample clock, so time spent
+    ///   *before* the site can never be attributed to it by the first
+    ///   in-site check (the symmetric over-attribution).
+    #[inline]
+    fn site_transition(slot: *const crate::runtime::Slot, old: u32, new: u32) {
+        let exp = EXPERIMENT.load(Ordering::Relaxed);
+        if exp == 0 || old == new {
+            return;
+        }
+        let target = (exp >> 32) as u32;
+        let pct = exp & 0xffff_ffff;
+        if old == target && new != target {
+            let now = preempt::rdtsc();
+            let last = LAST_SAMPLE_TSC.with(|c| c.replace(now));
+            if pct > 0 {
+                if last != 0 {
+                    // SAFETY: forwarded from the guard, which holds the on-CPU
+                    // actor's slot for its whole life (see `SiteGuard::slot`).
+                    unsafe { attribute(slot, now.saturating_sub(last), pct) };
+                } else {
+                    DISCARD_UNARMED_N.fetch_add(1, Ordering::Relaxed);
+                }
+            }
+        } else if new == target && old != target {
+            LAST_SAMPLE_TSC.with(|c| c.set(preempt::rdtsc()));
+        }
+    }
+
+    /// Resume-path hook (scheduler thread, actor off-CPU). Two duties:
+    ///
+    /// - If the last deschedule was a *real park*, time blocked absorbs any
+    ///   delay accrued meanwhile for free — Coz's blocked-thread rule, which
+    ///   keeps experiments from punishing actors for waiting. An actor that
+    ///   merely yielded (slice expiry) was runnable the whole time and keeps
+    ///   its debt: it must pay by spinning at its next check. Forgiving on
+    ///   every resume would make any yield-cadence actor delay-immune and
+    ///   experiments inert (found live on a 24-core run: nothing slowed).
+    /// - If the deschedule was a *yield* in the live experiment's target
+    ///   site, count the off-CPU gap it opened into the offcpu audit bucket
+    ///   (RFC 007: the located @50 deficit — runnable queue-wait is wall
+    ///   time in-site that on-CPU attribution correctly skips). Same-window
+    ///   only, enforced by the experiment epoch; measure-only.
+    /// - Arm this thread's sample clock so the first interval of the resume
+    ///   excludes scheduler time.
+    #[inline]
+    pub(crate) fn on_resume(slot: &crate::runtime::Slot) {
+        let (desched_tsc, desched_epoch) = slot.take_causal_desched();
+        if desched_tsc != 0 && desched_epoch == EXPERIMENT_EPOCH.load(Ordering::Relaxed) {
+            // Same epoch ⇒ no `begin()` since the stash; a nonzero word ⇒
+            // no `end()` either — the gap closed inside its own window.
+            let exp = EXPERIMENT.load(Ordering::Relaxed);
+            if exp != 0 {
+                let pct = exp & 0xffff_ffff;
+                let gap = preempt::rdtsc()
+                    .saturating_sub(desched_tsc)
+                    .min(MAX_SAMPLE_CYCLES);
+                OFFCPU_IN_SITE_CYCLES
+                    .fetch_add(gap.saturating_mul(pct) / 100, Ordering::Relaxed);
+                OFFCPU_IN_SITE_N.fetch_add(1, Ordering::Relaxed);
+            }
+        }
+        if slot.take_causal_parked() {
+            let global = GLOBAL_DELAY.load(Ordering::Relaxed);
+            let mine = slot.causal_delay();
+            if mine < global {
+                PARK_FORGIVEN_CYCLES.fetch_add(global - mine, Ordering::Relaxed);
+                slot.set_causal_delay(global);
+            }
+        }
+        LAST_SAMPLE_TSC.with(|c| c.set(preempt::rdtsc()));
+    }
+
+    /// Deschedule-path hook (scheduler side, same OS thread the actor just
+    /// ran on). If an experiment is live and the departing actor sits in the
+    /// target site, the sample tail `[last sample -> now]` is about to be
+    /// lost: nothing flushes it here, and `on_resume` re-arms the clock
+    /// before the actor runs again. Measure-only (RFC 007 deficit hunt) —
+    /// tally the would-be attribution into the park/yield drop buckets and
+    /// leave behaviour untouched. The interval is capped at
+    /// MAX_SAMPLE_CYCLES: past that the flush would have discarded it anyway
+    /// (counted separately). `now` includes the few hundred ns of scheduler
+    /// bookkeeping since the actor actually stopped — an acceptable
+    /// overcount for a diagnostic.
+    ///
+    /// Slice-expiry yields sample at the same checkpoint that deschedules
+    /// them, so their tails are ~zero by construction; a fat yield bucket
+    /// therefore points at explicit `yield_now` calls or requeued parks.
+    ///
+    /// Yields additionally stash the deschedule instant on the slot so
+    /// `on_resume` can count the runnable off-CPU gap (offcpu bucket).
+    pub(crate) fn on_deschedule(slot: &crate::runtime::Slot, real_park: bool) {
+        let exp = EXPERIMENT.load(Ordering::Relaxed);
+        if exp == 0 {
+            return;
+        }
+        let target = (exp >> 32) as u32;
+        let pct = exp & 0xffff_ffff;
+        if pct == 0 || slot.causal_site() != target {
+            return;
+        }
+        let now = preempt::rdtsc();
+        if !real_park {
+            // Runnable gap opens here; `on_resume` closes and counts it
+            // (offcpu bucket). Parks are excluded: blocked time is already
+            // represented by forgiveness, and blocked wall time is not
+            // queue-wait.
+            slot.set_causal_desched(now, EXPERIMENT_EPOCH.load(Ordering::Relaxed));
+        }
+        let last = LAST_SAMPLE_TSC.with(|c| c.get());
+        if last == 0 {
+            return;
+        }
+        let interval = now.saturating_sub(last).min(MAX_SAMPLE_CYCLES);
+        let would_be = interval.saturating_mul(pct) / 100;
+        if real_park {
+            DROP_PARK_N.fetch_add(1, Ordering::Relaxed);
+            DROP_PARK_CYCLES.fetch_add(would_be, Ordering::Relaxed);
+        } else {
+            DROP_YIELD_N.fetch_add(1, Ordering::Relaxed);
+            DROP_YIELD_CYCLES.fetch_add(would_be, Ordering::Relaxed);
+        }
+    }
+
+    // -----------------------------------------------------------------------
+    // Experiments
+    // -----------------------------------------------------------------------
+
+    fn begin(site: u32, pct: u32) {
+        EXPERIMENT_EPOCH.fetch_add(1, Ordering::Relaxed);
+        EXPERIMENT.store(((site as u64) << 32) | pct as u64, Ordering::Relaxed);
+    }
+
+    fn end() {
+        EXPERIMENT.store(0, Ordering::Relaxed);
+    }
+
+    /// Total virtual delay injected so far, in TSC cycles.
+    pub fn global_delay_cycles() -> u64 {
+        GLOBAL_DELAY.load(Ordering::Relaxed)
+    }
+
+    /// Cumulative ledger-audit totals since startup (RFC 007 deficit hunt).
+    /// All monotone; window a span by snapshotting before/after and taking
+    /// `delta_since`. Cycle fields are in would-be-injected delta terms so
+    /// they compare directly against `injected_cycles`.
+    #[derive(Clone, Copy, Debug, Default)]
+    pub struct LedgerCounters {
+        pub spin_absorbed_cycles: u64,
+        pub park_forgiven_cycles: u64,
+        pub drop_park_cycles: u64,
+        pub drop_park_n: u64,
+        pub drop_yield_cycles: u64,
+        pub drop_yield_n: u64,
+        pub discard_overmax_cycles: u64,
+        pub discard_overmax_n: u64,
+        pub discard_unarmed_n: u64,
+        pub offcpu_in_site_cycles: u64,
+        pub offcpu_in_site_n: u64,
+    }
+
+    impl LedgerCounters {
+        /// Field-wise difference against an earlier snapshot.
+        pub fn delta_since(&self, before: &LedgerCounters) -> LedgerCounters {
+            LedgerCounters {
+                spin_absorbed_cycles: self
+                    .spin_absorbed_cycles
+                    .saturating_sub(before.spin_absorbed_cycles),
+                park_forgiven_cycles: self
+                    .park_forgiven_cycles
+                    .saturating_sub(before.park_forgiven_cycles),
+                drop_park_cycles: self.drop_park_cycles.saturating_sub(before.drop_park_cycles),
+                drop_park_n: self.drop_park_n.saturating_sub(before.drop_park_n),
+                drop_yield_cycles: self
+                    .drop_yield_cycles
+                    .saturating_sub(before.drop_yield_cycles),
+                drop_yield_n: self.drop_yield_n.saturating_sub(before.drop_yield_n),
+                discard_overmax_cycles: self
+                    .discard_overmax_cycles
+                    .saturating_sub(before.discard_overmax_cycles),
+                discard_overmax_n: self.discard_overmax_n.saturating_sub(before.discard_overmax_n),
+                discard_unarmed_n: self.discard_unarmed_n.saturating_sub(before.discard_unarmed_n),
+                offcpu_in_site_cycles: self
+                    .offcpu_in_site_cycles
+                    .saturating_sub(before.offcpu_in_site_cycles),
+                offcpu_in_site_n: self.offcpu_in_site_n.saturating_sub(before.offcpu_in_site_n),
+            }
+        }
+    }
+
+    /// Snapshot the cumulative audit counters.
+    pub fn ledger_counters() -> LedgerCounters {
+        LedgerCounters {
+            spin_absorbed_cycles: SPIN_ABSORBED_CYCLES.load(Ordering::Relaxed),
+            park_forgiven_cycles: PARK_FORGIVEN_CYCLES.load(Ordering::Relaxed),
+            drop_park_cycles: DROP_PARK_CYCLES.load(Ordering::Relaxed),
+            drop_park_n: DROP_PARK_N.load(Ordering::Relaxed),
+            drop_yield_cycles: DROP_YIELD_CYCLES.load(Ordering::Relaxed),
+            drop_yield_n: DROP_YIELD_N.load(Ordering::Relaxed),
+            discard_overmax_cycles: DISCARD_OVERMAX_CYCLES.load(Ordering::Relaxed),
+            discard_overmax_n: DISCARD_OVERMAX_N.load(Ordering::Relaxed),
+            discard_unarmed_n: DISCARD_UNARMED_N.load(Ordering::Relaxed),
+            offcpu_in_site_cycles: OFFCPU_IN_SITE_CYCLES.load(Ordering::Relaxed),
+            offcpu_in_site_n: OFFCPU_IN_SITE_N.load(Ordering::Relaxed),
+        }
+    }
+
+    /// Absorbed-delay ledger of the on-CPU actor (testing/introspection).
+    pub fn my_absorbed_delay_cycles() -> u64 {
+        let slot = preempt::current_slot_ptr();
+        if slot.is_null() {
+            return 0;
+        }
+        // SAFETY: on-CPU actor's slot; see `check_cancelled`.
+        unsafe { (*slot).causal_delay() }
+    }
+
+    /// Test support: start an experiment targeting `site_name` at `pct`%
+    /// virtual speedup. Registers the site if needed.
+    pub fn begin_experiment_for_test(name: &'static str, pct: u32) {
+        begin(site_id(name), pct);
+    }
+
+    /// Test support: stop the running experiment.
+    pub fn end_experiment_for_test() {
+        end();
+    }
+
+    /// Test support: grow the global delay ledger directly, as if target-site
+    /// samples had attributed `cycles` — deterministic driver for the timer
+    /// virtual-time tests. Calibrates the TSC eagerly so conversion later
+    /// never stalls a scheduler loop.
+    pub fn inject_delay_cycles_for_test(cycles: u64) {
+        let _ = tsc_hz();
+        GLOBAL_DELAY.fetch_add(cycles, Ordering::Relaxed);
+    }
+
+    /// Convert ledger cycles to wall time at the measured TSC rate.
+    pub fn cycles_to_duration(cycles: u64) -> Duration {
+        Duration::from_secs_f64(cycles as f64 / tsc_hz())
+    }
+
+    /// Controller parameters: which speedups to try per site, and the
+    /// experiment/cooldown windows.
+    pub struct ExperimentPlan {
+        pub speedups_pct: Vec<u32>,
+        pub experiment: Duration,
+        pub cooldown: Duration,
+    }
+
+    impl Default for ExperimentPlan {
+        fn default() -> Self {
+            ExperimentPlan {
+                speedups_pct: vec![0, 25, 50],
+                experiment: Duration::from_millis(500),
+                cooldown: Duration::from_millis(100),
+            }
+        }
+    }
+
+    /// One completed experiment cell.
+    #[derive(Default)]
+    pub struct ExperimentResult {
+        pub site: String,
+        pub speedup_pct: u32,
+        pub duration: Duration,
+        /// Progress-point deltas over the window, `(name, count)`.
+        pub deltas: Vec<(String, u64)>,
+        /// Virtual delay injected during the window (cycles).
+        pub injected_cycles: u64,
+        // Ledger-audit deltas over the window (RFC 007 deficit hunt); see
+        // `LedgerCounters` for field semantics. `spin_absorbed_cycles > 0`
+        // in a 0% cell means the window paid debt left over from an earlier
+        // one — the baseline-contamination signature.
+        pub spin_absorbed_cycles: u64,
+        pub park_forgiven_cycles: u64,
+        pub drop_park_cycles: u64,
+        pub drop_park_n: u64,
+        pub drop_yield_cycles: u64,
+        pub drop_yield_n: u64,
+        pub discard_overmax_cycles: u64,
+        pub discard_overmax_n: u64,
+        pub discard_unarmed_n: u64,
+        pub offcpu_in_site_cycles: u64,
+        pub offcpu_in_site_n: u64,
+    }
+
+    /// Run the plan synchronously on the calling (OS) thread: for every
+    /// registered site × speedup, run one experiment window and record
+    /// progress-point deltas, with a cooldown between cells. Sites and
+    /// progress points must already be registered (the workload has to be
+    /// running); the caller owns workload start/stop.
+    ///
+    /// v1 controller: exhaustive sweep, fixed windows, no adaptive site
+    /// selection or confidence stopping (jar Q5).
+    ///
+    /// Callable from a plain OS thread *or* from inside an actor: sleeping
+    /// parks the green thread when we're on one (so no scheduler thread is
+    /// blocked), and falls back to `thread::sleep` otherwise.
+    pub fn run_experiments(plan: &ExperimentPlan) -> Vec<ExperimentResult> {
+        fn controller_sleep(d: Duration) {
+            if preempt::current_slot_ptr().is_null() {
+                std::thread::sleep(d);
+            } else {
+                // Wall-anchored: the controller's window/cooldown sleeps
+                // *define* the experiment's wall length; letting them chase
+                // the delay it is itself injecting would stretch every window
+                // (observed ~2x at 50% speedup). Deltas are rate-normalized
+                // either way — this fixes cost, not bias.
+                crate::scheduler::sleep_wall(d);
+            }
+        }
+        // Calibrate before any window so report rendering never has to sleep.
+        let _ = tsc_hz();
+        let site_list: Vec<(u32, String)> = {
+            let v = lock_unpoisoned(sites());
+            v.iter()
+                .enumerate()
+                .map(|(i, n)| ((i + 1) as u32, (*n).to_string()))
+                .collect()
+        };
+        let mut out = Vec::new();
+        for (sid, sname) in &site_list {
+            for &pct in &plan.speedups_pct {
+                let before = progress_snapshot();
+                let injected_before = global_delay_cycles();
+                let audit_before = ledger_counters();
+                let t0 = Instant::now();
+                begin(*sid, pct);
+                controller_sleep(plan.experiment);
+                end();
+                // Snapshot immediately: injection and spin freeze at `end()`
+                // (checks gate on the experiment word), but forgiveness does
+                // not — a later snapshot would leak cooldown wakes into the
+                // window.
+                let audit = ledger_counters().delta_since(&audit_before);
+                let elapsed = t0.elapsed();
+                let after = progress_snapshot();
+                let deltas = after
+                    .iter()
+                    .map(|(n, c)| {
+                        let b = before
+                            .iter()
+                            .find(|(bn, _)| bn == n)
+                            .map(|(_, bc)| *bc)
+                            .unwrap_or(0);
+                        (n.clone(), c.saturating_sub(b))
+                    })
+                    .collect();
+                out.push(ExperimentResult {
+                    site: sname.clone(),
+                    speedup_pct: pct,
+                    duration: elapsed,
+                    deltas,
+                    injected_cycles: global_delay_cycles() - injected_before,
+                    spin_absorbed_cycles: audit.spin_absorbed_cycles,
+                    park_forgiven_cycles: audit.park_forgiven_cycles,
+                    drop_park_cycles: audit.drop_park_cycles,
+                    drop_park_n: audit.drop_park_n,
+                    drop_yield_cycles: audit.drop_yield_cycles,
+                    drop_yield_n: audit.drop_yield_n,
+                    discard_overmax_cycles: audit.discard_overmax_cycles,
+                    discard_overmax_n: audit.discard_overmax_n,
+                    discard_unarmed_n: audit.discard_unarmed_n,
+                    offcpu_in_site_cycles: audit.offcpu_in_site_cycles,
+                    offcpu_in_site_n: audit.offcpu_in_site_n,
+                });
+                controller_sleep(plan.cooldown);
+            }
+        }
+        out
+    }
+
+    // -----------------------------------------------------------------------
+    // Reports
+    // -----------------------------------------------------------------------
+
+    /// Measured TSC frequency (Hz), calibrated once. The crate-wide 3 GHz
+    /// constant is fine for the *relative* timeslice check, but report
+    /// normalisation divides wall time by injected time, so a 20% Hz error
+    /// skews every impact number — measured live: a 3.7 GHz box inflated all
+    /// baselines uniformly. Calibrated against `Instant` over ~50ms on first
+    /// use; `run_experiments` triggers it before its first window (using the
+    /// park-aware sleep, so no scheduler thread is blocked when called from
+    /// an actor).
+    static TSC_HZ_MEASURED: OnceLock<f64> = OnceLock::new();
+
+    /// Measured TSC frequency in Hz. Calibrates on first call (~50ms).
+    pub fn tsc_hz() -> f64 {
+        *TSC_HZ_MEASURED.get_or_init(|| {
+            let c0 = preempt::rdtsc();
+            let t0 = Instant::now();
+            let d = Duration::from_millis(50);
+            if preempt::current_slot_ptr().is_null() {
+                std::thread::sleep(d);
+            } else {
+                // Wall-anchored: calibration divides TSC delta by *wall*
+                // elapsed; a virtual sleep dilated by concurrent injection
+                // would still measure correctly (elapsed() is wall) but
+                // waste window time — and must never depend on the ledger
+                // it exists to convert.
+                crate::scheduler::sleep_wall(d);
+            }
+            (preempt::rdtsc().wrapping_sub(c0)) as f64 / t0.elapsed().as_secs_f64()
+        })
+    }
+
+    /// Normalized rate for one cell: count over the *virtual* window
+    /// (wall − injected) — Coz's normalization: injected delay does not
+    /// exist in the virtual timeline. A bottleneck site keeps its raw count
+    /// while shrinking the divisor → positive impact; a fully overlapped
+    /// site loses count proportionally → ~zero.
+    fn normalized_rate(r: &ExperimentResult, point: &str) -> Option<f64> {
+        let count = r.deltas.iter().find(|(n, _)| n == point).map(|(_, c)| *c)?;
+        let injected_secs = r.injected_cycles as f64 / tsc_hz();
+        let virtual_secs = (r.duration.as_secs_f64() - injected_secs).max(1e-9);
+        Some(count as f64 / virtual_secs)
+    }
+
+    /// Impact of virtually speeding up `site` by `speedup_pct` on progress
+    /// point `point`, in percent relative to that site's own 0% baseline
+    /// cell. `None` if either cell or the point is missing, or the baseline
+    /// rate is zero. This is the machine-readable form of the summary's
+    /// "vs baseline" column, for programmatic checks (CI, examples).
+    pub fn impact_pct(
+        results: &[ExperimentResult],
+        site: &str,
+        speedup_pct: u32,
+        point: &str,
+    ) -> Option<f64> {
+        let cell = results
+            .iter()
+            .find(|r| r.site == site && r.speedup_pct == speedup_pct)?;
+        let base = results.iter().find(|r| r.site == site && r.speedup_pct == 0)?;
+        let rate = normalized_rate(cell, point)?;
+        let b = normalized_rate(base, point)?;
+        if b <= 0.0 {
+            return None;
+        }
+        Some((rate / b - 1.0) * 100.0)
+    }
+
+    /// Human-readable summary: per (site, progress point), the throughput at
+    /// each virtual speedup and the change relative to that site's own 0%
+    /// baseline. A near-zero column across speedups means: optimising this
+    /// site buys you nothing — the RFC's headline answer.
+    ///
+    /// Ends with a one-line fidelity note (RFC 007 Validation): reported
+    /// impacts are conservative — attribution counts on-CPU site time only,
+    /// so runnable queue-wait inside the site (the located @50 "deficit",
+    /// eff ≈ 0.93 live) is never injected and gains are lower bounds; site
+    /// *rankings* are unaffected.
+    pub fn render_summary(results: &[ExperimentResult]) -> String {
+        use std::fmt::Write;
+        let mut s = String::new();
+        let _ = writeln!(s, "== smarm causal profile ==");
+        let mut sites_seen: Vec<&str> = Vec::new();
+        for r in results {
+            if !sites_seen.contains(&r.site.as_str()) {
+                sites_seen.push(&r.site);
+            }
+        }
+        for site in sites_seen {
+            let _ = writeln!(s, "site {site}");
+            for r in results.iter().filter(|r| r.site == site) {
+                for (name, _) in &r.deltas {
+                    let rate = match normalized_rate(r, name) {
+                        Some(x) => x,
+                        None => continue,
+                    };
+                    let rel = impact_pct(results, site, r.speedup_pct, name)
+                        .map(|p| format!("{p:+.1}%"))
+                        .unwrap_or_else(|| "n/a".to_string());
+                    let _ = writeln!(
+                        s,
+                        "  speedup {:>3}%  {name:<24} {rate:>12.1}/s  vs baseline {rel}  (injected {:.1}ms)",
+                        r.speedup_pct,
+                        r.injected_cycles as f64 / tsc_hz() * 1e3
+                    );
+                }
+            }
+        }
+        if !results.is_empty() {
+            let _ = writeln!(
+                s,
+                "note: impacts are lower bounds — site time counts on-CPU only (runnable queue-wait is not attributed); rankings unaffected"
+            );
+        }
+        s
+    }
+
+    /// Ledger-audit companion to `render_summary` (RFC 007 deficit hunt):
+    /// per cell, where the window's virtual delay went — born (injected),
+    /// paid (absorbed), waived (forgiven at wake) — and the attribution the
+    /// sampler lost: tails dropped at parks/yields inside the target site,
+    /// plus clamp discards. Cycle columns in ms at the calibrated TSC rate.
+    /// `absorbed` above `injected` in a cell (0% especially) means it paid
+    /// debt left over from earlier windows.
+    pub fn render_ledger_audit(results: &[ExperimentResult]) -> String {
+        use std::fmt::Write;
+        let hz = tsc_hz();
+        let ms = |c: u64| c as f64 / hz * 1e3;
+        let mut s = String::new();
+        let _ = writeln!(s, "== smarm causal ledger audit ==");
+        for r in results {
+            let _ = writeln!(
+                s,
+                "site {:<22} @{:>2}%  injected {:>7.1}ms  absorbed {:>7.1}ms  forgiven {:>7.1}ms  \
+                 drop park {:>6.2}ms/{:<4} yield {:>6.2}ms/{:<4} offcpu {:>6.2}ms/{:<5} \
+                 discard >max {:>6.2}ms/{:<3} unarmed {}",
+                r.site,
+                r.speedup_pct,
+                ms(r.injected_cycles),
+                ms(r.spin_absorbed_cycles),
+                ms(r.park_forgiven_cycles),
+                ms(r.drop_park_cycles),
+                r.drop_park_n,
+                ms(r.drop_yield_cycles),
+                r.drop_yield_n,
+                ms(r.offcpu_in_site_cycles),
+                r.offcpu_in_site_n,
+                ms(r.discard_overmax_cycles),
+                r.discard_overmax_n,
+                r.discard_unarmed_n
+            );
+        }
+        s
+    }
+
+    /// Coz-compatible profile text (`profile.coz`), so Coz's existing plot
+    /// tooling renders our experiments — the RFC's "don't build a UI" call.
+    pub fn render_coz(results: &[ExperimentResult]) -> String {
+        use std::fmt::Write;
+        let mut s = String::new();
+        let _ = writeln!(s, "startup\ttime=0");
+        for r in results {
+            let _ = writeln!(
+                s,
+                "experiment\tselected={}\tspeedup={:.2}\tduration={}\tselected-samples=1",
+                r.site,
+                r.speedup_pct as f64 / 100.0,
+                r.duration.as_nanos()
+            );
+            for (name, count) in &r.deltas {
+                let _ = writeln!(s, "throughput-point\tname={name}\tdelta={count}");
+            }
+        }
+        s
+    }
+}
+
+#[cfg(feature = "smarm-causal")]
+pub use inner::*;
+
+/// Mark one unit of useful work complete at a named throughput progress
+/// point (RFC 007). One Relaxed increment when `smarm-causal` is on; nothing
+/// at all when it's off.
+#[cfg(feature = "smarm-causal")]
+#[macro_export]
+macro_rules! progress {
+    ($name:literal) => {{
+        static __SMARM_PP: ::std::sync::OnceLock<&'static $crate::causal::ProgressPoint> =
+            ::std::sync::OnceLock::new();
+        __SMARM_PP
+            .get_or_init(|| $crate::causal::register_progress($name))
+            .bump();
+    }};
+}
+
+#[cfg(not(feature = "smarm-causal"))]
+#[macro_export]
+macro_rules! progress {
+    ($name:literal) => {{}};
+}
+
+/// Enter a named causal-profiling site for the current actor; the returned
+/// guard exits it (restoring any enclosing site) on drop. Site identity is
+/// stored in the actor's slot, so it survives preemption and migration.
+/// Expands to a unit no-op without `smarm-causal`.
+#[cfg(feature = "smarm-causal")]
+#[macro_export]
+macro_rules! causal_site {
+    ($name:literal) => {{
+        static __SMARM_SITE: ::std::sync::OnceLock<u32> = ::std::sync::OnceLock::new();
+        $crate::causal::SiteGuard::enter(*__SMARM_SITE.get_or_init(|| $crate::causal::site_id($name)))
+    }};
+}
+
+#[cfg(not(feature = "smarm-causal"))]
+#[macro_export]
+macro_rules! causal_site {
+    ($name:literal) => {
+        ()
+    };
+}

src/channel.rs

@@ -1,38 +1,102 @@
-//! Unbounded MPSC channels.
+//! Unbounded multi-producer, single-consumer channels: how actors talk to
+//! each other.
 //!
-//! Inner state is `Arc<RawMutex<Inner<T>>>` so channels can be sent across OS
-//! threads (required for the multi-scheduler runtime where a sender and
-//! receiver may run on different scheduler threads simultaneously).
+//! A channel is a queue with a typed [`Sender`] on one end and a typed
+//! [`Receiver`] on the other. Any number of actors can hold a clone of the
+//! `Sender` and push messages onto the same queue; exactly one [`Receiver`]
+//! reads them back out, in the order they arrived. This is the basic wiring
+//! smarm's other actor primitives (`gen_server`, `pg`, the registry) are all
+//! built out of, and it is directly usable on its own for a worker that just
+//! needs an inbox.
 //!
-//! ## Why `RawMutex` (Channel class), not `std::sync::Mutex`
+//! ## A first channel
 //!
-//! An actor holding a guard with preemption *enabled* can be timesliced
-//! inside the critical section and resume on a different OS thread — the
-//! pthread mutex would then be released from a thread that didn't lock it,
-//! which is UB (Linux futexes happen to tolerate it, but it's not
-//! guaranteed). `RawMutex` disables preemption for the guard's span and is
-//! cross-thread-release sound by construction, closing the hole. It also
-//! cannot poison. Channel locks form their own [`LockClass::Channel`]
-//! (raw_mutex.rs): they may be taken under a cold (Leaf) lock — finalize and
-//! `monitor()` clone senders that live in slots — but nothing may be locked
-//! under them, which the debug build enforces. `recv_match` runs its user
-//! predicate under this lock: keep it cheap, pure, and channel-free.
+//! ```
+//! use smarm::{channel, run, spawn};
 //!
-//! Semantics:
-//!   - Senders are clonable; the last sender drop closes the channel.
-//!   - `Receiver::recv` on an empty open channel parks the receiver.
-//!   - `Receiver::recv` on an empty closed channel returns `Err(RecvError)`.
-//!   - `Sender::send` on an open channel always succeeds.
-//!   - `Sender::send` on a closed channel (receiver dropped) returns
-//!     `Err(SendError(value))`.
-//!   - When a send pushes to a previously empty queue and a receiver is
-//!     parked, the receiver is unparked.
+//! run(|| {
+//!     let (tx, rx) = channel::<u64>();
+//!
+//!     let worker = spawn(move || {
+//!         // Blocks until a message arrives.
+//!         let n = rx.recv().unwrap();
+//!         assert_eq!(n, 42);
+//!
+//!         // Once every Sender is dropped, recv() reports the channel closed
+//!         // instead of blocking forever.
+//!         assert!(rx.recv().is_err());
+//!     });
+//!
+//!     tx.send(42).unwrap();
+//!     drop(tx); // last sender gone: the channel is now closed
+//!     worker.join().unwrap();
+//! });
+//! ```
+//!
+//! ## Sending
+//!
+//! [`Sender`] is cheaply clonable: hand a clone to every actor that needs to
+//! push messages into this queue. The channel stays open as long as at least
+//! one clone exists; [`Sender::send`] never blocks and always succeeds while
+//! the channel is open, since the queue is unbounded. Once the [`Receiver`]
+//! has been dropped, `send` returns the message back to you in
+//! [`SendError`] instead of delivering it.
+//!
+//! ## Receiving
+//!
+//! There is exactly one [`Receiver`] per channel (it is not clonable).
+//! [`Receiver::recv`] returns the next message in arrival order, parking the
+//! calling actor if the queue is currently empty. Once every `Sender` has
+//! been dropped and the queue has been drained, `recv` stops parking and
+//! returns [`RecvError`] instead, so a receiver never blocks forever waiting
+//! on senders that are never coming back.
+//!
+//! Beyond plain `recv`, three variants cover the common needs:
+//!
+//! - [`Receiver::try_recv`]: never parks: reports an empty-but-open channel
+//!   as `Ok(None)` instead of waiting.
+//! - [`Receiver::recv_timeout`]: parks, but gives up and returns
+//!   [`RecvTimeoutError::Timeout`] if no message arrives before a deadline.
+//! - [`Receiver::recv_match`] / [`Receiver::try_recv_match`]: selective
+//!   receive. Instead of taking whatever is at the front of the queue, pick
+//!   out the first message matching a predicate, leaving the rest queued in
+//!   order. Handy for an actor that wants to prioritise one kind of message
+//!   over others already waiting.
+//!
+//! ## Waiting on several channels: `select`
+//!
+//! [`select`] parks an actor across several receivers at once and reports
+//! the index of the first one that is ready (has a message queued, or has
+//! been closed). [`select_timeout`] adds a deadline, the way `recv_timeout`
+//! does for a single channel. See their docs for the full contract,
+//! including the priority-order and no-fairness guarantee.
+//!
+//! ## Implementation notes
+//!
+//! The queue and its bookkeeping live behind `Arc<RawMutex<Inner<T>>>`
+//! rather than a `std::sync::Mutex`, so that a channel can be freely shared
+//! and sent across the OS threads backing the multi-scheduler runtime.
+//! `RawMutex` matters here for a subtler reason too: an ordinary pthread
+//! mutex can be released from a different OS thread than the one that took
+//! it (smarm's preemption can migrate a timesliced actor between scheduler
+//! threads mid-critical-section), and doing that to a `std::sync::Mutex` is
+//! undefined behavior. `RawMutex` disables preemption for the guard's short
+//! lifetime instead, so the release always happens on the thread that
+//! acquired it, and it has no poisoning to worry about besides. Channel
+//! locks are cheap and are never held across another lock acquisition or a
+//! blocking call; the predicate passed to `recv_match` runs under this lock,
+//! which is why it needs to stay cheap, pure, and must not call back into
+//! the same channel.
 
 use crate::pid::Pid;
 use crate::raw_mutex::RawMutex;
 use std::collections::VecDeque;
 use std::sync::Arc;
 
+/// Create a new channel and return its `(Sender, Receiver)` halves.
+///
+/// The channel is unbounded (no capacity limit) and open until every
+/// `Sender` has been dropped.
 pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
     let inner = Arc::new(RawMutex::new_channel(Inner {
         queue: VecDeque::new(),
@@ -45,29 +109,40 @@ pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
 
 struct Inner<T> {
     queue: VecDeque<T>,
-    /// The parked receiver's `(pid, park-epoch)`. The epoch is the slot
-    /// word's runtime-wide wait identity (see slot_state.rs): wakers call
-    /// `unpark_at(pid, epoch)`, so an entry left over from an already-woken
-    /// wait — a `select` loser arm, a satisfied `recv_timeout`'s timer — is
-    /// inert: the wake fails the word's epoch CAS and no-ops. This replaces
-    /// the old per-channel `cur_wait`/`next_wait_seq`/`timed_out` trio: wait
-    /// identity now exists exactly once, in the slot word.
+    /// The parked receiver's `(pid, park-epoch)`, if one is currently
+    /// waiting. The epoch identifies exactly which wait this is, so a waker
+    /// left over from a wait that already ended (a losing `select` arm, a
+    /// `recv_timeout` whose timer fired after it was already satisfied) is
+    /// inert and does nothing when it fires.
     parked_receiver: Option<(Pid, u32)>,
     senders: usize,
     receiver_alive: bool,
 }
 
+/// The sending half of a channel, created by [`channel`]. Clonable: every
+/// clone pushes onto the same queue, and the channel stays open as long as
+/// any clone is alive. Dropping the last `Sender` closes the channel, which
+/// wakes a parked [`Receiver`] so it can observe the closure.
 pub struct Sender<T> {
     inner: Arc<RawMutex<Inner<T>>>,
 }
 
+/// The receiving half of a channel, created by [`channel`]. Not clonable:
+/// a channel has exactly one receiver. Reads messages in the order they
+/// were sent, via [`recv`](Receiver::recv) and its variants.
 pub struct Receiver<T> {
     inner: Arc<RawMutex<Inner<T>>>,
 }
 
+/// Returned by [`Sender::send`] when the channel's [`Receiver`] has already
+/// been dropped. Carries the message back so it is never silently lost;
+/// recover it with `.0` or by matching.
 #[derive(Debug, PartialEq, Eq)]
 pub struct SendError<T>(pub T);
 
+/// Returned by [`Receiver::recv`] (and the other receive methods, in their
+/// own error types) when the channel is closed: every `Sender` has been
+/// dropped and no message is left queued.
 #[derive(Debug, PartialEq, Eq, Clone, Copy)]
 pub struct RecvError;
 
@@ -84,8 +159,8 @@ impl std::error::Error for RecvError {}
 pub enum RecvTimeoutError {
     /// The deadline passed with no message available.
     Timeout,
-    /// All senders dropped with no message available — the bounded analogue
-    /// of [`RecvError`].
+    /// Every sender was dropped with no message available. The
+    /// timeout-aware counterpart of plain [`RecvError`].
     Disconnected,
 }
 
@@ -115,8 +190,8 @@ impl<T> Drop for Sender<T> {
             // Wake the parked receiver on the last sender drop regardless of
             // whether the queue is empty. A plain `recv` only ever parks on an
             // empty queue (so this is unchanged for it), but a selective
-            // `recv_match` may be parked on a *non-empty* queue holding only
-            // non-matching messages — it must wake to observe closure and
+            // `recv_match` may be parked on a non-empty queue holding only
+            // non-matching messages. It must wake to observe closure and
             // return Err rather than sleep forever.
             if g.senders == 0 {
                 g.parked_receiver.take()
@@ -132,19 +207,37 @@ impl<T> Drop for Sender<T> {
 
 impl<T> Drop for Receiver<T> {
     fn drop(&mut self) {
-        self.inner.lock().receiver_alive = false;
+        // The only consumer is gone: queued messages can never be delivered.
+        // Drop them now instead of leaving them queued until the last Sender
+        // happens to go away, which can be long after this receiver's owner
+        // has exited if some other part of the runtime is still holding a
+        // clone of the Sender. Draining runs each queued message's own drop
+        // glue, which matters for a gen_server call: dropping a queued call
+        // envelope drops its reply channel too, which wakes the caller with
+        // an error instead of leaving it parked forever. Drain under the
+        // lock, then run the drops after releasing it, since a message's
+        // drop glue may itself touch a different channel or the scheduler.
+        let drained = {
+            let mut g = self.inner.lock();
+            g.receiver_alive = false;
+            std::mem::take(&mut g.queue)
+        };
+        drop(drained);
     }
 }
 
 impl<T> Sender<T> {
-    /// Number of messages currently queued behind this channel. Introspection
-    /// only (RFC 016 mailbox depth); takes the channel lock, so callers reach
-    /// it under the registry Leaf (Leaf → Channel) via the erased probe in
-    /// `registry.rs`, never on a hot path.
+    /// Number of messages currently queued and not yet received. For
+    /// introspection and monitoring; takes the channel's internal lock, so
+    /// avoid calling it from a hot path.
     pub(crate) fn queued_len(&self) -> usize {
         self.inner.lock().queue.len()
     }
 
+    /// Push `value` onto the channel. Succeeds unconditionally as long as
+    /// the [`Receiver`] is still alive: the queue has no capacity limit, so
+    /// this never blocks and never fails except when the channel is closed,
+    /// in which case `value` comes back in [`SendError`].
     pub fn send(&self, value: T) -> Result<(), SendError<T>> {
         let unpark = {
             let mut g = self.inner.lock();
@@ -165,6 +258,10 @@ impl<T> Sender<T> {
 }
 
 impl<T> Receiver<T> {
+    /// Block until a message is available and return it. Messages come back
+    /// in the order they were sent. If the queue is empty and every
+    /// [`Sender`] has already been dropped, returns [`RecvError`] instead of
+    /// blocking forever.
     pub fn recv(&self) -> Result<T, RecvError> {
         loop {
             {
@@ -176,54 +273,50 @@ impl<T> Receiver<T> {
                 if g.senders == 0 {
                     return Err(RecvError);
                 }
-                let me = crate::actor::current_pid()
-                    .expect("recv() called outside an actor");
+                let me = match crate::actor::current_pid() {
+                    Some(me) => me,
+                    None => panic!("smarm: recv() called outside an actor"),
+                };
                 debug_assert!(
                     g.parked_receiver.is_none_or(|(p, _)| p == me),
                     "channel has more than one receiver"
                 );
-                // begin_wait is lock-free — legal under the Channel lock;
+                // begin_wait is lock-free, so it's legal under the Channel lock;
                 // registering in the same critical section makes the epoch
                 // atomic with the senders' view of the registration.
                 g.parked_receiver = Some((me, crate::scheduler::begin_wait()));
                 crate::te!(crate::trace::Event::RecvPark(me));
             }
-            // Release the lock before parking — the unparker will need it.
+            // Release the lock before parking: the unparker will need it.
             crate::scheduler::park_current();
-            // Woken up — record it before looping to check the queue.
-            crate::te!(crate::trace::Event::RecvWake(crate::actor::current_pid().unwrap()));
+            // Woken up. Record it before looping to check the queue.
+            crate::te!(crate::trace::Event::RecvWake(match crate::actor::current_pid() {
+                Some(p) => p,
+                None => panic!("smarm: RecvWake outside an actor (core corrupt)"),
+            }));
         }
     }
 
-    /// Bounded receive: like [`recv`](Self::recv), but gives up once
-    /// `timeout` has elapsed, returning [`RecvTimeoutError::Timeout`].
+    /// Like [`recv`](Self::recv), but gives up and returns
+    /// [`RecvTimeoutError::Timeout`] if no message has arrived by the time
+    /// `timeout` elapses.
     ///
-    /// Built on the same timer machinery as `Mutex::lock_timeout`: the wait
-    /// registers a `WaitTimeout` entry stamped with the wait's park-epoch;
-    /// on expiry the channel (as the
-    /// [`TimerTarget`](crate::timer::TimerTarget)) checks whether *this*
-    /// wait is still parked and, only then, cancels it. A wake that races
-    /// the deadline resolves message-first: if a message is available when
-    /// the receiver runs, it is delivered even if the timer had already
-    /// fired. A satisfied or abandoned wait leaves its timer entry to expire
-    /// as a no-op (registration gone; epoch consumed), per the
-    /// no-cancellation convention in `timer.rs`.
+    /// If a message arrives at essentially the same moment the deadline
+    /// passes, the message wins: you get `Ok` rather than `Timeout`. If
+    /// every sender is dropped before a message arrives or the deadline
+    /// passes, you get [`RecvTimeoutError::Disconnected`].
     ///
-    /// The wake is classified from state alone — wakes are precise (the only
-    /// stamped wakers of this wait are a send, the last-sender drop, and the
-    /// timer; a stop wake unwinds out of `park_current` and never reaches
-    /// the classification), so: message queued → `Ok`; `senders == 0` →
-    /// `Disconnected`; neither → it was the timer → `Timeout`.
-    ///
-    /// `Duration::ZERO` is a valid timeout: it parks until the immediately-
-    /// due timer is drained, then reports `Timeout` unless a message was
-    /// already queued.
+    /// `Duration::ZERO` is a valid timeout: it still gives any
+    /// already-queued message a chance to be returned, and only then
+    /// reports `Timeout`.
     pub fn recv_timeout(&self, timeout: std::time::Duration) -> Result<T, RecvTimeoutError>
     where
         T: Send + 'static,
     {
-        let me = crate::actor::current_pid()
-            .expect("recv_timeout() called outside an actor");
+        let me = match crate::actor::current_pid() {
+            Some(me) => me,
+            None => panic!("smarm: recv_timeout() called outside an actor"),
+        };
 
         // Fast path + wait registration, one critical section.
         let epoch;
@@ -247,14 +340,17 @@ impl<T> Receiver<T> {
 
         // Arm the timer after releasing the channel lock (insert takes the
         // timers lock; never nest under a Channel lock). A send or even the
-        // timer itself may unpark us before we park — the RunningNotified
+        // timer itself may unpark us before we park; the runtime's wake
         // protocol makes the park below return immediately in that case.
         let deadline = crate::timer::deadline_from_now(timeout);
         let target: std::sync::Arc<dyn crate::timer::TimerTarget> = self.inner.clone();
         crate::scheduler::insert_wait_timer(deadline, me, target, epoch);
 
         crate::scheduler::park_current();
-        crate::te!(crate::trace::Event::RecvWake(crate::actor::current_pid().unwrap()));
+        crate::te!(crate::trace::Event::RecvWake(match crate::actor::current_pid() {
+            Some(p) => p,
+            None => panic!("smarm: RecvWake outside an actor (core corrupt)"),
+        }));
         let mut g = self.inner.lock();
         if let Some(v) = g.queue.pop_front() {
             crate::preempt::note_message_received();
@@ -266,16 +362,23 @@ impl<T> Receiver<T> {
         Err(RecvTimeoutError::Timeout)
     }
 
-    /// Selective receive: remove and return the first queued message for which
-    /// `pred` holds, leaving the rest in arrival order. If no queued message
-    /// matches, parks and re-scans on every send (a selective receiver may park
-    /// on a *non-empty* queue). Returns `Err(RecvError)` only once the channel
-    /// is closed and no queued message matches.
+    /// Selective receive: find and return the first queued message for
+    /// which `pred` returns `true`, leaving every other message in the
+    /// queue untouched and in order. Useful when an actor's inbox mixes
+    /// message kinds and it wants to handle one kind out of turn, without
+    /// discarding the rest.
     ///
-    /// `pred` is run while the channel lock is held: keep it cheap and pure,
-    /// and do not call back into this channel from inside it. It is modelled as
-    /// `Fn` (not `FnMut`) deliberately — it is re-run from scratch on every
-    /// scan, so a stateful predicate would observe surprising re-counting.
+    /// If nothing queued matches, this blocks and re-checks every time a new
+    /// message arrives, the same way [`recv`](Self::recv) blocks on an empty
+    /// queue: a selective receiver can be waiting even while the queue holds
+    /// messages, just none that match yet. Returns [`RecvError`] only once
+    /// the channel is closed and still nothing matches.
+    ///
+    /// `pred` runs while the channel is locked, so keep it cheap, side
+    /// effect free, and make sure it never calls back into this same
+    /// channel. It takes `&T` and is called fresh on every scan (not `FnMut`
+    /// with running state), so it should judge each message purely on its
+    /// own content.
     pub fn recv_match<F>(&self, pred: F) -> Result<T, RecvError>
     where
         F: Fn(&T) -> bool,
@@ -283,17 +386,23 @@ impl<T> Receiver<T> {
         loop {
             {
                 let mut g = self.inner.lock();
-                if let Some(i) = g.queue.iter().position(|v| pred(v)) {
+                if let Some(i) = g.queue.iter().position(&pred) {
                     // position() found it, so remove() returns Some.
                     crate::preempt::note_message_received();
-                    return Ok(g.queue.remove(i).unwrap());
+                    let v = match g.queue.remove(i) {
+                        Some(v) => v,
+                        None => panic!("smarm: channel queue.remove after position (logic bug)"),
+                    };
+                    return Ok(v);
                 }
                 if g.senders == 0 {
                     // Closed and nothing queued can ever match.
                     return Err(RecvError);
                 }
-                let me = crate::actor::current_pid()
-                    .expect("recv_match() called outside an actor");
+                let me = match crate::actor::current_pid() {
+                    Some(me) => me,
+                    None => panic!("smarm: recv_match() called outside an actor"),
+                };
                 debug_assert!(
                     g.parked_receiver.is_none_or(|(p, _)| p == me),
                     "channel has more than one receiver"
@@ -301,24 +410,33 @@ impl<T> Receiver<T> {
                 g.parked_receiver = Some((me, crate::scheduler::begin_wait()));
                 crate::te!(crate::trace::Event::RecvPark(me));
             }
-            // Release the lock before parking — the unparker will need it.
+            // Release the lock before parking: the unparker will need it.
             crate::scheduler::park_current();
-            crate::te!(crate::trace::Event::RecvWake(crate::actor::current_pid().unwrap()));
+            crate::te!(crate::trace::Event::RecvWake(match crate::actor::current_pid() {
+                Some(p) => p,
+                None => panic!("smarm: RecvWake outside an actor (core corrupt)"),
+            }));
         }
     }
 
-    /// Non-blocking selective receive. `Ok(Some(v))` if a queued message
-    /// matched `pred` (removed, rest left in order), `Ok(None)` if the channel
-    /// is open but nothing matched, `Err(RecvError)` if closed and nothing
-    /// matched. Same predicate contract as [`recv_match`](Self::recv_match).
+    /// The non-blocking counterpart of [`recv_match`](Self::recv_match):
+    /// returns immediately either way. `Ok(Some(v))` if a queued message
+    /// matched `pred` (removed; the rest stay queued in order), `Ok(None)`
+    /// if the channel is open but nothing currently matches, `Err(RecvError)`
+    /// if the channel is closed and nothing matches. Same predicate contract
+    /// as `recv_match`.
     pub fn try_recv_match<F>(&self, pred: F) -> Result<Option<T>, RecvError>
     where
         F: Fn(&T) -> bool,
     {
         let mut g = self.inner.lock();
-        if let Some(i) = g.queue.iter().position(|v| pred(v)) {
+        if let Some(i) = g.queue.iter().position(&pred) {
             crate::preempt::note_message_received();
-            return Ok(Some(g.queue.remove(i).unwrap()));
+            let v = match g.queue.remove(i) {
+                Some(v) => v,
+                None => panic!("smarm: channel queue.remove after position (logic bug)"),
+            };
+            return Ok(Some(v));
         }
         if g.senders == 0 {
             return Err(RecvError);
@@ -326,8 +444,10 @@ impl<T> Receiver<T> {
         Ok(None)
     }
 
-    /// Non-blocking. `Ok(Some(v))` if a message was available, `Ok(None)` if
-    /// the channel is empty but open, `Err(RecvError)` if closed and drained.
+    /// The non-blocking counterpart of [`recv`](Self::recv): returns
+    /// immediately either way. `Ok(Some(v))` if a message was queued,
+    /// `Ok(None)` if the channel is open but currently empty, `Err(RecvError)`
+    /// if the channel is closed and the queue is drained.
     pub fn try_recv(&self) -> Result<Option<T>, RecvError> {
         let mut g = self.inner.lock();
         if let Some(v) = g.queue.pop_front() {
@@ -342,18 +462,18 @@ impl<T> Receiver<T> {
 }
 
 // ---------------------------------------------------------------------------
-// TimerTarget — the expiry half of recv_timeout
+// TimerTarget: the expiry half of recv_timeout
 // ---------------------------------------------------------------------------
 
 impl<T: Send + 'static> crate::timer::TimerTarget for RawMutex<Inner<T>> {
     fn on_timeout(&self, pid: Pid, epoch: u32) {
         // Cancel the wait only if THIS wait (epoch match) is still
         // registered. If a sender already took `parked_receiver`, the
-        // receiver is waking with a message — message wins, the timer
+        // receiver is waking with a message: message wins, the timer
         // no-ops. If a later wait by the same receiver is registered, the
-        // epoch mismatches — stale entry, no-op. (The unpark_at would fail
-        // its word CAS in either case anyway; checking under the lock keeps
-        // the registration bookkeeping exact.)
+        // epoch mismatches: stale entry, no-op. (unpark_at would fail its
+        // internal check in either case anyway; checking under the lock
+        // keeps the registration bookkeeping exact.)
         let unpark = {
             let mut g = self.lock();
             if g.parked_receiver == Some((pid, epoch)) {
@@ -363,7 +483,7 @@ impl<T: Send + 'static> crate::timer::TimerTarget for RawMutex<Inner<T>> {
                 false
             }
         };
-        // Unpark outside the channel lock — it may take the run-queue lock;
+        // Unpark outside the channel lock: it may take the run-queue lock;
         // legal under a Channel lock, but pointless to nest.
         if unpark {
             crate::scheduler::unpark_at(pid, epoch);
@@ -372,7 +492,7 @@ impl<T: Send + 'static> crate::timer::TimerTarget for RawMutex<Inner<T>> {
 }
 
 // ---------------------------------------------------------------------------
-// select — ready-index wait over multiple receivers
+// select: ready-index wait over multiple receivers
 // ---------------------------------------------------------------------------
 
 pub(crate) mod sealed {
@@ -380,33 +500,34 @@ pub(crate) mod sealed {
 }
 impl<T> sealed::Sealed for Receiver<T> {}
 
-/// An arm of a [`select`]. Implemented by [`Receiver`]; sealed, because the
-/// registration contract below is part of the runtime's wake protocol.
+/// An arm of a [`select`]: something you can wait on alongside other arms
+/// and be told when it becomes ready. Implemented by [`Receiver`]; sealed
+/// (cannot be implemented outside this crate), since the registration
+/// contract below is part of the runtime's internal wake protocol.
 ///
 /// Contract (all under the arm's own lock): `sel_register` checks-or-
-/// registers atomically — if the arm is ready it does NOT register and
+/// registers atomically. If the arm is ready it does not register and
 /// returns `Ok(false)`; otherwise it publishes `(pid, epoch)` where its
 /// wakers will find it and returns `Ok(true)`. "Ready" means a receive
-/// would not park: a message is queued, or the arm is closed. `Err` means
+/// would not block: a message is queued, or the arm is closed. `Err` means
 /// the arm could not register at all (only fd arms can fail; channel
-/// registration is infallible) — the wait must be retired and earlier
+/// registration always succeeds), and the wait must be retired and earlier
 /// eager-cleanup arms unregistered.
 pub trait Selectable: sealed::Sealed {
     #[doc(hidden)]
     fn sel_register(&self, pid: Pid, epoch: u32) -> std::io::Result<bool>;
     #[doc(hidden)]
     fn sel_ready(&self) -> bool;
-    /// Remove this arm's `(pid, epoch)` registration if — and only if — it
-    /// is still in place. Default no-op: a losing channel arm's stale
-    /// registration is inert (its wakers die at the epoch CAS; the next
-    /// wait overwrites the slot). Fd arms override this: their staleness
-    /// poisons the fd (waiters entry + kernel-side ONESHOT registration)
-    /// and needs an eager cleanup pass.
+    /// Remove this arm's `(pid, epoch)` registration if, and only if, it is
+    /// still in place. Default no-op: a losing channel arm's stale
+    /// registration is harmless and self-cleans. Fd arms override this:
+    /// their staleness would otherwise leave the fd unusable for future
+    /// selects, so they need an eager cleanup pass.
     #[doc(hidden)]
     fn sel_unregister(&self, _pid: Pid, _epoch: u32) {}
     /// Whether this arm requires the eager cleanup pass at all. Gates the
-    /// post-wake `sel_unregister` sweep so channel-only selects keep
-    /// today's zero-cancellation hot path.
+    /// post-wake `sel_unregister` sweep so channel-only selects keep their
+    /// cheap, cleanup-free path.
     #[doc(hidden)]
     fn sel_eager_cleanup(&self) -> bool {
         false
@@ -433,50 +554,54 @@ impl<T> Selectable for Receiver<T> {
     }
 }
 
-/// Park on every arm at once; return the index of the first ready one.
+/// Wait on several channels at once and return the index of the first one
+/// that is ready, instead of blocking on just one with [`Receiver::recv`].
 ///
-/// "Ready" means a receive on that arm would not park: a message is queued,
-/// or the arm is **closed** (so the caller's `try_recv` observes the
-/// disconnect — a dead arm is an event, not a hang). The caller consumes the
-/// arm itself, typically via [`Receiver::try_recv`]; single-receiver
-/// channels guarantee nothing can steal the message in between.
+/// "Ready" means a receive on that arm would not block: a message is
+/// queued, or the arm is closed (so the caller's own `try_recv` observes
+/// the disconnect: a dead arm is something to react to, not something to
+/// hang on). `select` only tells you which arm is ready; read the actual
+/// message yourself, typically with [`Receiver::try_recv`] on that arm.
 ///
-/// A closed arm stays ready *forever*: once its disconnect has been
-/// observed, drop it from the arm set — under priority order it would
-/// otherwise win every subsequent call and starve every higher-indexed arm.
+/// A closed arm stays ready forever. Once you have observed its disconnect,
+/// drop it from the arm set you pass in next time: otherwise, under the
+/// priority order below, it would win every subsequent call and starve
+/// every arm listed after it.
 ///
-/// Arms are scanned **in order**: index 0 is the highest priority, both on
-/// the immediate-ready path and after a wake. This is a documented
-/// guarantee (compose like BEAM receive clauses: put control channels
-/// first), not an accident — and therefore there is NO fairness promise; a
-/// saturated arm 0 starves arm 1 by design.
+/// Arms are checked **in order**: index 0 is the highest priority, both
+/// when checking immediately and after being woken. This is a deliberate,
+/// documented guarantee, not an accident of implementation: put a control
+/// or shutdown channel first so it is always noticed promptly. The
+/// flip side is that there is **no fairness guarantee**: a busy arm 0 can
+/// starve arm 1 indefinitely by design.
 ///
-/// One actor may select on a channel and later `recv` on it (or select on
-/// overlapping sets) freely. What stays illegal is what was always illegal:
-/// two *different* actors receiving on one channel.
+/// One actor can `select` on a channel and later plain `recv` on it (or
+/// `select` again on an overlapping set of arms) with no restriction. What
+/// stays illegal is what was always illegal for a channel: two *different*
+/// actors receiving on the same one.
 ///
-/// Built on the consuming-wake protocol (see slot_state.rs): all arms are
-/// registered under one wait epoch; the winning wake consumes it, so losing
-/// arms' registrations are inert and need no cancellation pass — they
-/// self-clean at their wakers' failed CAS, or get overwritten by this
-/// receiver's next wait on that channel.
-///
-/// Panics if `arms` is empty, when called outside an actor, or if an fd
-/// arm fails to register (EBADF, EMFILE, a second waiter on one fd —
-/// see [`try_select`] for the fallible form; channel-only selects cannot
-/// fail).
+/// Panics if `arms` is empty, if called outside an actor, or if an fd arm
+/// fails to register (see [`try_select`] for the fallible form; a
+/// channel-only `select` can never fail).
 pub fn select(arms: &[&dyn Selectable]) -> usize {
-    try_select(arms).expect("select(): fd arm failed to register (use try_select)")
+    match try_select(arms) {
+        Ok(i) => i,
+        Err(e) => panic!("smarm: select() fd arm failed to register (use try_select): {e}"),
+    }
 }
 
-/// [`select`], fallible: `Err` when an arm fails to register (only fd
-/// arms can — EBADF, EMFILE on the epoll set, or a second waiter on an
-/// fd that already has one). On `Err` the wait is fully retired and no
+/// The fallible form of [`select`]: `Err` when an arm fails to register.
+/// Only fd arms can fail this way (for example, the file descriptor is
+/// invalid, or something else is already waiting on it); a channel-only
+/// select can never fail. On `Err` the wait is fully retired and no
 /// registration is left behind: every arm registered before the failing
 /// one has been unregistered.
 pub fn try_select(arms: &[&dyn Selectable]) -> std::io::Result<usize> {
     assert!(!arms.is_empty(), "select() on an empty arm list");
-    let me = crate::actor::current_pid().expect("select() called outside an actor");
+    let me = match crate::actor::current_pid() {
+        Some(me) => me,
+        None => panic!("smarm: select() called outside an actor"),
+    };
     loop {
         let epoch = crate::scheduler::begin_wait();
         if let Some(i) = register_arms(me, epoch, arms)? {
@@ -484,13 +609,12 @@ pub fn try_select(arms: &[&dyn Selectable]) -> std::io::Result<usize> {
         }
 
         // Stale fd registrations are not harmless (a losing fd arm's
-        // waiters entry poisons the fd with AlreadyExists and its
-        // kernel-side ONESHOT registration can fire arbitrarily late), so
-        // selects containing fd arms run an eager cleanup pass after the
-        // park — including when a terminal stop unwinds out of it, via
-        // the guard. Channel-only selects skip all of it: `eager` is
-        // false, the guard is disarmed, and the loser-arm self-cleaning
-        // story is unchanged.
+        // leftover registration can make the fd unusable for the next
+        // select until a kernel event happens to clear it), so selects
+        // containing fd arms run an eager cleanup pass after the park,
+        // including when a terminal stop unwinds out of it, via the guard.
+        // Channel-only selects skip all of it: `eager` is false, the guard
+        // is disarmed, and the loser-arm self-cleaning story is unchanged.
         let eager = arms.iter().any(|a| a.sel_eager_cleanup());
         let mut guard = UnregisterGuard { arms, me, epoch, armed: eager };
 
@@ -503,22 +627,22 @@ pub fn try_select(arms: &[&dyn Selectable]) -> std::io::Result<usize> {
         drop(guard);
 
         // Woken precisely: an arm's send (message) or last-sender drop
-        // (closure) consumed our epoch, and both leave their arm ready —
-        // return the first one, in priority order (which may be a
+        // (closure) is what woke us, and both leave their arm ready.
+        // Return the first ready one, in priority order (which may be a
         // different, higher-priority arm than the one that woke us; its
         // message stays queued and re-reports ready on the next call).
         // Fd arms classify by a fresh zero-timeout poll, so they too are
-        // a pure function of state — independent of the registration the
-        // cleanup pass just removed.
+        // a pure function of current state, independent of the
+        // registration the cleanup pass just removed.
         for (i, arm) in arms.iter().enumerate() {
             if arm.sel_ready() {
                 return Ok(i);
             }
         }
-        // Unreachable by protocol (a stop wake unwinds out of
-        // park_current). Defensive: re-open the wait and re-register —
-        // stale own-registrations are overwritten (channels) or were
-        // removed by the cleanup pass above (fds).
+        // Unreachable in practice (a stop wake unwinds out of
+        // park_current before we get here). Defensive: re-open the wait
+        // and re-register; stale own-registrations are overwritten
+        // (channels) or were removed by the cleanup pass above (fds).
     }
 }
 
@@ -533,11 +657,11 @@ fn unregister_arms(arms: &[&dyn Selectable], me: Pid, epoch: u32) {
     }
 }
 
-/// Stop-unwind twin of the explicit cleanup pass: a terminal stop unwinds
-/// out of `park_current`, and a registered fd arm must not outlive its
-/// actor (the generalization of `wait_fd`'s `Dereg`). Disarmed on the
-/// normal path after the explicit pass runs; never armed when no fd arm
-/// registered, keeping the channel-only path guard-free in effect.
+// Stop-unwind twin of the explicit cleanup pass: a terminal stop unwinds
+// out of `park_current`, and a registered fd arm must not outlive its
+// actor. Disarmed on the normal path after the explicit pass runs; never
+// armed when no fd arm is registered, keeping the channel-only path
+// guard-free in effect.
 struct UnregisterGuard<'a> {
     arms: &'a [&'a dyn Selectable],
     me: Pid,
@@ -553,20 +677,16 @@ impl Drop for UnregisterGuard<'_> {
     }
 }
 
-/// The registration pass shared by [`select`] and [`select_timeout`]:
-/// check-or-register each arm, in priority order, each atomically under its
-/// own lock. Cross-arm atomicity is unnecessary: an arm becoming ready
-/// right after its registration wakes the caller through the protocol (the
-/// prep-to-park window is closed by RunningNotified).
-///
-/// `Ok(Some(i))` = arm `i` was ready, the pass stopped, and the wait has
-/// been RETIRED (no park may follow): earlier arms hold live-epoch
-/// registrations, so earlier *fd* arms are unregistered eagerly, then the
-/// epoch is bumped, a landed notification eaten, and a pending stop
-/// re-observed — without which a stale arm wake could fault a later
-/// one-shot park. `Err` = an arm failed to register; identical unwind
-/// (earlier fd arms unregistered, wait retired). `Ok(None)` = every arm
-/// registered; the caller parks.
+// The registration pass shared by `select` and `select_timeout`: check-or-
+// register each arm, in priority order, each atomically under its own lock.
+// Cross-arm atomicity is unnecessary: an arm becoming ready right after its
+// registration still wakes the caller through the normal wake path.
+//
+// `Ok(Some(i))` = arm `i` was already ready, the pass stopped, and the wait
+// has been fully retired (no park may follow): earlier fd arms are
+// unregistered eagerly so none are left dangling. `Err` = an arm failed to
+// register; same unwind (earlier fd arms unregistered, wait retired).
+// `Ok(None)` = every arm registered successfully; the caller parks.
 fn register_arms(
     me: Pid,
     epoch: u32,
@@ -590,10 +710,10 @@ fn register_arms(
     Ok(None)
 }
 
-/// The [`select_timeout`] timer target: stateless, because precise wakes
-/// make classification a pure function of channel state. The entry is
-/// stamped with the select's epoch; if an arm already won, this unpark dies
-/// at the word's epoch CAS (the no-cancellation convention in `timer.rs`).
+// The `select_timeout` timer target: stateless, because a wake's cause can
+// always be read back off plain channel state (an arm ready, or not). If
+// an arm already won before the deadline, this timer's fire is simply
+// ignored, the way any other stale wakeup is.
 struct SelectTimeout;
 impl crate::timer::TimerTarget for SelectTimeout {
     fn on_timeout(&self, pid: Pid, epoch: u32) {
@@ -601,60 +721,60 @@ impl crate::timer::TimerTarget for SelectTimeout {
     }
 }
 
-/// [`select`] with a deadline: returns `Some(index)` like `select`, or
-/// `None` once `timeout` elapses with no arm ready.
+/// Like [`select`], but gives up and returns `None` if no arm becomes
+/// ready before `timeout` elapses.
 ///
-/// All of `select`'s semantics carry over (priority order, closed arms
-/// permanently ready, no fairness promise). The timeout is one more stamped
-/// waker on the same wait epoch — nothing is registered in any arm for it,
-/// so there is nothing to cancel or leak: an arm winning leaves the timer
-/// entry to expire as a stale-epoch no-op; the timer winning leaves the
-/// arms' registrations to self-clean exactly as a `select` loser's would.
+/// All of `select`'s semantics carry over: arms are still checked in
+/// priority order, a closed arm is still permanently ready, and there is
+/// still no fairness guarantee across arms. A message that arrives at
+/// essentially the same moment the deadline passes still wins, the same
+/// way [`Receiver::recv_timeout`] resolves that race.
 ///
-/// The wake is classified from state alone (wakes are precise): some arm
-/// ready → `Some` of the first, in priority order; none ready → the timer
-/// was the only remaining stamped waker → `None`. A message that races the
-/// deadline resolves message-first, as `recv_timeout` does.
+/// `Duration::ZERO` is a valid timeout: it still gives an already-ready arm
+/// a chance to be reported before falling through to `None`.
 ///
-/// `Duration::ZERO` is a valid timeout: it parks until the immediately-due
-/// timer is drained, then reports `None` unless an arm was already ready.
-///
-/// Panics if `arms` is empty, when called outside an actor, or if an fd
-/// arm fails to register (see [`try_select_timeout`] for the fallible
-/// form; channel-only selects cannot fail).
+/// Panics if `arms` is empty, if called outside an actor, or if an fd arm
+/// fails to register (see [`try_select_timeout`] for the fallible form; a
+/// channel-only select can never fail).
 pub fn select_timeout(
     arms: &[&dyn Selectable],
     timeout: std::time::Duration,
 ) -> Option<usize> {
-    try_select_timeout(arms, timeout)
-        .expect("select_timeout(): fd arm failed to register (use try_select_timeout)")
+    match try_select_timeout(arms, timeout) {
+        Ok(r) => r,
+        Err(e) => panic!(
+            "smarm: select_timeout() fd arm failed to register (use try_select_timeout): {e}"
+        ),
+    }
 }
 
-/// [`select_timeout`], fallible: `Err` when an arm fails to register
-/// (only fd arms can). On `Err` the wait is fully retired and no
-/// registration — arm-side or kernel-side — is left behind.
+/// The fallible form of [`select_timeout`]: `Err` when an arm fails to
+/// register (only fd arms can). On `Err` the wait is fully retired and no
+/// registration is left behind on any arm.
 pub fn try_select_timeout(
     arms: &[&dyn Selectable],
     timeout: std::time::Duration,
 ) -> std::io::Result<Option<usize>> {
     assert!(!arms.is_empty(), "select_timeout() on an empty arm list");
-    let me = crate::actor::current_pid().expect("select_timeout() called outside an actor");
+    let me = match crate::actor::current_pid() {
+        Some(me) => me,
+        None => panic!("smarm: select_timeout() called outside an actor"),
+    };
     let epoch = crate::scheduler::begin_wait();
     if let Some(i) = register_arms(me, epoch, arms)? {
         return Ok(Some(i)); // ready now: the timer was never armed
     }
 
-    // Arm the timer after the registration pass, outside every Channel
-    // lock (insert takes the timers lock).
+    // Arm the timer after the registration pass, outside every channel
+    // lock (inserting a timer takes the timers lock).
     let deadline = crate::timer::deadline_from_now(timeout);
     let target: std::sync::Arc<dyn crate::timer::TimerTarget> = std::sync::Arc::new(SelectTimeout);
     crate::scheduler::insert_wait_timer(deadline, me, target, epoch);
 
-    // Same eager-cleanup story as `try_select`: the timer arm needs none
-    // (stateless, stale entries die at the epoch CAS), channel arms need
-    // none, fd arms do — and a timer win in particular leaves every fd
-    // arm's registration behind, which without this pass would poison
-    // those fds until a kernel event happened to fire.
+    // Same eager-cleanup story as `try_select`: a timer win in particular
+    // leaves every fd arm's registration behind, which without this pass
+    // would leave those fds unusable until a kernel event happened to
+    // clear them.
     let eager = arms.iter().any(|a| a.sel_eager_cleanup());
     let mut guard = UnregisterGuard { arms, me, epoch, armed: eager };
 

src/context.rs

@@ -94,10 +94,28 @@ unsafe extern "C" fn switch_to_actor_asm() {
 }
 
 /// Resume the actor whose sp is in `ACTOR_SP`. Returns when the actor yields.
+///
+/// # Safety
+///
+/// The caller must be running on a scheduler thread with a valid actor stack
+/// pointer installed in `ACTOR_SP` — either by `init_actor_stack` (first
+/// resume) or by a prior `switch_to_scheduler` (subsequent resumes). Resuming
+/// with an unset or stale `ACTOR_SP` transfers control to an arbitrary address.
+/// Must not be called from within an actor (only the scheduler side may resume).
 pub unsafe fn switch_to_actor() {
     unsafe { switch_to_actor_asm() };
 }
 
+/// Yield from the running actor back to its scheduler thread. Returns when the
+/// actor is next resumed via [`switch_to_actor`].
+///
+/// # Safety
+///
+/// The caller must be running on an actor stack that was entered through
+/// [`switch_to_actor`], so that `SCHEDULER_SP` holds the live saved stack
+/// pointer of the scheduler side. Calling this from the scheduler thread, or
+/// before any actor has been resumed, transfers control to an arbitrary
+/// address.
 #[unsafe(naked)]
 pub unsafe extern "C" fn switch_to_scheduler() {
     core::arch::naked_asm!(

src/gen_server.rs

@@ -528,7 +528,10 @@ impl<G: GenServer> TimerHandle<G> {
     /// [`TimerId`] you can pass to `cancel`. Timer fires arrive before info and
     /// inbox messages.
     pub fn arm_after(&self, after: Duration, msg: G::Timer) -> TimerId {
-        let mut reg = self.reg.lock().unwrap();
+        let mut reg = match self.reg.lock() {
+            Ok(g) => g,
+            Err(e) => panic!("smarm: gen_server reg lock poisoned (core corrupt): {e}"),
+        };
         let local = reg.mint();
         // The fire thunk carries the local id so the loop can retire the entry
         // on dispatch; `msg` is moved in (no `Clone` needed for one-shots).
@@ -552,7 +555,10 @@ impl<G: GenServer> TimerHandle<G> {
     where
         G::Timer: Clone,
     {
-        let mut reg = self.reg.lock().unwrap();
+        let mut reg = match self.reg.lock() {
+            Ok(g) => g,
+            Err(e) => panic!("smarm: gen_server reg lock poisoned (core corrupt): {e}"),
+        };
         let local = reg.mint();
         // A live periodic must keep the system arm open so the next tick can be
         // re-armed; the first periodic installs the loop's re-arm sender.
@@ -577,7 +583,10 @@ impl<G: GenServer> TimerHandle<G> {
     /// stops re-arming: the entry is removed so the loop will not schedule
     /// another tick even if the pending one already escaped onto the channel.
     pub fn cancel(&self, id: TimerId) -> bool {
-        let mut reg = self.reg.lock().unwrap();
+        let mut reg = match self.reg.lock() {
+            Ok(g) => g,
+            Err(e) => panic!("smarm: gen_server reg lock poisoned (core corrupt): {e}"),
+        };
         if let Some(sub) = reg.oneshots.remove(&id) {
             return cancel_timer(sub);
         }
@@ -587,7 +596,7 @@ impl<G: GenServer> TimerHandle<G> {
                 reg.rearm_tx = None;
             }
             debug_assert!(
-                reg.rearm_tx.is_some() == !reg.periodics.is_empty(),
+                reg.rearm_tx.is_some() != reg.periodics.is_empty(),
                 "rearm_tx must be Some iff periodics is non-empty"
             );
             return beat;
@@ -846,7 +855,10 @@ fn server_loop<G: GenServer>(
     impl<G: GenServer> Drop for Terminate<G> {
         fn drop(&mut self) {
             {
-                let mut reg = self.1.lock().unwrap();
+                let mut reg = match self.1.lock() {
+                    Ok(g) => g,
+                    Err(e) => panic!("smarm: gen_server reg lock poisoned (core corrupt): {e}"),
+                };
                 for (_, sub) in reg.oneshots.drain() {
                     cancel_timer(sub);
                 }
@@ -995,7 +1007,10 @@ fn server_loop<G: GenServer>(
                         // The one-shot fired: retire its registry entry so the
                         // live set tracks only still-pending timers, then
                         // dispatch.
-                        reg.lock().unwrap().oneshots.remove(&id);
+                        match reg.lock() {
+                            Ok(mut g) => { g.oneshots.remove(&id); }
+                            Err(e) => panic!("smarm: gen_server reg lock poisoned (core corrupt): {e}"),
+                        }
                         guard.0.handle_timer(msg);
                         reset_idle(&mut idle_deadline);
                     }
@@ -1006,16 +1021,20 @@ fn server_loop<G: GenServer>(
                         // so the period is measured from fire-handling and a
                         // handler that cancels stops the just-armed instance.
                         let msg = {
-                            let mut g = reg.lock().unwrap();
+                            let mut g = match reg.lock() {
+                                Ok(g) => g,
+                                Err(e) => panic!("smarm: gen_server reg lock poisoned (core corrupt): {e}"),
+                            };
                             let r = &mut *g;
                             if let Some(p) = r.periodics.get_mut(&id) {
                                 let every = p.every;
                                 let msg = (p.make)();
-                                let tx = r
-                                    .rearm_tx
-                                    .as_ref()
-                                    .expect("a live periodic keeps rearm_tx Some")
-                                    .clone();
+                                let tx = match r.rearm_tx.as_ref() {
+                                    Some(tx) => tx.clone(),
+                                    None => panic!(
+                                        "smarm: live periodic without rearm_tx (logic bug)"
+                                    ),
+                                };
                                 p.live = send_after_to(every, tx, Sys::Tick(id));
                                 Some(msg)
                             } else {

src/gen_statem.rs

@@ -14,13 +14,13 @@
 //! [`on_start`](Machine::on_start), then one [`handle`](Machine::handle) per
 //! inbox event — mirroring `gen_server`'s spawn/teardown idioms.
 //!
-//! The [`gen_statem!`](crate::gen_statem) macro is the authoring surface: it
+//! The [`gen_statem!`](crate::gen_statem!) macro is the authoring surface: it
 //! *generates* a `Machine` impl from per-state handler blocks. Edge-validity is
 //! not a separate check — it falls out of the generated total `match (state,
 //! event)` under denied lints (a forgotten or duplicated pair is a compile
 //! error), so no proc-macro is needed. See `examples/gen_statem_macro.rs` for
-//! the macro form and `examples/gen_statem_fused.rs` for the hand-written shape
-//! it expands to.
+//! the macro form and `examples/gen_statem_expanded.rs` for the hand-written
+//! shape it expands to.
 //!
 //! ## The unified event
 //!
@@ -38,19 +38,51 @@
 //! `gen_server` call round-trip, but with the reply handle riding *inside* the
 //! user's own event so a handler can answer it.
 //!
-//! [`Resolution::Postpone`] and the timeout arming on [`Cx`] are part of the
-//! type surface but are not yet wired up.
+//! ## Timeouts
+//!
+//! Two timeout flavours, both armed from a handler through [`Cx`] and both
+//! surfacing back as ordinary **events** the machine matches in its `on State`
+//! arms — unlike `gen_server`, which routes fires to a separate handler:
+//!
+//! - [`cx.state_timeout(d)`](Cx::state_timeout) fires a `state_timeout` event
+//!   after `d` *in the current state*, and is auto-reset on any state change.
+//!   Only one is ever pending; arming again replaces it.
+//! - [`cx.timeout(name, d)`](Cx::timeout) fires a `timeout(name)` event after
+//!   `d`, **survives** state changes, and is keyed by `name` so several can be
+//!   in flight and each is independently [`cancel_timeout`](Cx::cancel_timeout)led.
+//!
+//! Both ride the same timer min-heap as `gen_server` (no separate timer
+//! mechanism): a fire lands on the loop's own system channel — above the inbox,
+//! so a timeout can't be starved by inbox traffic — and the loop turns it into
+//! the corresponding internal event and runs it through the normal `handle`
+//! dispatch.
+//!
+//! ## Postpone
+//!
+//! A `=> postpone` row defers the current event untouched, to be replayed after
+//! the next **real transition**. The deferred event — a `cast`, `call`, or
+//! `info` — moves whole onto a loop-side FIFO queue; a postponed `call` keeps
+//! its [`Reply`] handle and is answered by whichever later state handles the
+//! replay. On a transition the queue drains in order through the normal
+//! [`handle`](Machine::handle) dispatch in the new state, ahead of any further
+//! inbox or timer event; a replayed event may postpone again (it re-queues for
+//! the next transition). See the macro docs for the row surface and [`Step`]
+//! for how a postpone surfaces to the loop.
 
-use crate::channel::{channel, Receiver, Sender};
+use crate::channel::{channel, select, Receiver, Sender};
 use crate::pid::Pid;
-use crate::scheduler::spawn as spawn_actor;
+use crate::scheduler::{cancel_timer, send_after_to, spawn as spawn_actor};
+use crate::timer::TimerId;
+use std::collections::{HashMap, VecDeque};
 use std::marker::PhantomData;
+use std::sync::{Arc, Mutex};
+use std::time::Duration;
 
 // ---------------------------------------------------------------------------
 // Machine
 // ---------------------------------------------------------------------------
 
-/// A finite state machine driven by the [`statem`](crate::gen_statem) loop.
+/// A finite state machine driven by the [`statem`](mod@crate::gen_statem) loop.
 ///
 /// The implementor owns its current state tag and its persistent data. It is
 /// the **sole writer** of the state cell (the loop never touches it): both
@@ -59,28 +91,45 @@ use std::marker::PhantomData;
 /// [`handle`](Self::handle) per event.
 pub trait Machine: Send + 'static {
     /// The single payload this machine's inbox carries — the user's `cast` and
-    /// `call` enums folded together with the runtime's internal events. See the
-    /// module docs.
+    /// `call` enums folded together with the runtime's internal events
+    /// (`state_timeout`, `timeout(name)`, and an `Info` wrapper). See the module
+    /// docs.
     type Ev: Send + 'static;
 
+    /// Wrap a fired state-timeout into this machine's event. The loop calls this
+    /// when the pending state-timeout fires, then runs the result through
+    /// [`handle`](Self::handle) like any other event. The macro generates it as
+    /// `Ev::StateTimeout`.
+    fn state_timeout_ev() -> Self::Ev;
+
+    /// Wrap a fired named timeout into this machine's event, carrying the name
+    /// it was armed under. The macro generates it as `Ev::Timeout(name)`.
+    fn timeout_ev(name: &'static str) -> Self::Ev;
+
     /// Runs once inside the actor before the first event. The canonical body
     /// runs the `enter` arm for the initial state.
     fn on_start(&mut self, cx: &mut Cx<Self::Ev>);
 
-    /// React to one event. The body matches `(state, event)`, performs side
-    /// effects / replies, and ends each arm in a [`Resolution`] — typically a
-    /// state tag via `Tag.into()` (transition, or "stay" when it equals the
-    /// current tag). On a transition the body sets the state cell and runs the
-    /// new state's `enter`.
-    fn handle(&mut self, ev: Self::Ev, cx: &mut Cx<Self::Ev>);
+    /// React to one event, returning a [`Step`] the loop acts on. The body
+    /// first routes a `postpone` row (handing the event back untouched as
+    /// [`Step::Postponed`]); otherwise it matches `(state, event)`, performs
+    /// side effects / replies, and ends each arm in a [`Resolution`] — typically
+    /// a state tag via `Tag.into()` (transition, or "stay" when it equals the
+    /// current tag). On a transition the body sets the state cell, runs the new
+    /// state's `enter`, and returns [`Step::Transitioned`]; a stay or unmatched
+    /// event returns [`Step::Stayed`].
+    fn handle(&mut self, ev: Self::Ev, cx: &mut Cx<Self::Ev>) -> Step<Self::Ev>;
 }
 
 // ---------------------------------------------------------------------------
 // Resolution
 // ---------------------------------------------------------------------------
 
-/// The outcome of handling one event, before the loop/handler acts on it:
-/// dispatch reduces to `(state, event) -> Resolution<State>`.
+/// The outcome of the **consuming** dispatch — the by-value `match (state,
+/// event)` — before the apply-tail acts on it: `(state, event) ->
+/// Resolution<State>`. Postpone is *not* here: a deferred event never reaches
+/// this match (it is routed out first; see [`Step`] and the macro's two-phase
+/// `handle`), so the only outcomes are a target state or "unhandled".
 ///
 /// [`From<S>`](From) is why a bare state tag works as an arm tail and why
 /// "stay" needs no keyword — `Tag.into()` is `To(Tag)`, and the handler treats
@@ -89,10 +138,6 @@ pub enum Resolution<S> {
     /// End in state `s`. A **transition** when `s != current` (set the cell,
     /// run `enter`); a **stay** when `s == current` (no `enter`).
     To(S),
-    /// Defer the current event onto the postpone queue, to be replayed after
-    /// the next real transition. Produced by `cx.postpone()`; present
-    /// here for forward-compatibility but not yet generated.
-    Postpone,
     /// No arm matched `(state, event)`: log-and-drop via
     /// [`Cx::on_unhandled`], mirroring `gen_server`'s handling of unexpected
     /// messages.
@@ -105,31 +150,187 @@ impl<S> From<S> for Resolution<S> {
     }
 }
 
+/// What one [`handle`](Machine::handle) call did, as the loop needs to see it.
+/// Unlike [`Resolution`] (internal to the consuming match), this is the
+/// loop-visible result, because the loop must know two things the match alone
+/// doesn't surface: whether an event was **deferred** (so the loop owns it for
+/// the postpone queue) and whether a **real transition** happened (so the loop
+/// replays that queue). The three cases are mutually exclusive.
+pub enum Step<Ev> {
+    /// The event was deferred by a `postpone` row and handed back untouched.
+    /// The loop pushes it onto the postpone queue; no state change occurred.
+    Postponed(Ev),
+    /// A real transition happened (`enter` for the new state has already run).
+    /// The loop replays the postpone queue in the new state.
+    Transitioned,
+    /// Handled with no transition — a stay or an unmatched event. Nothing is
+    /// deferred and the postpone queue is left as-is.
+    Stayed,
+}
+
+// ---------------------------------------------------------------------------
+// Cx — the per-handler context
+// ---------------------------------------------------------------------------
+
+// ---------------------------------------------------------------------------
+// Sys — the loop's internal timer-fire channel
+// ---------------------------------------------------------------------------
+
+/// A timer fire landing on the loop's own system channel, selected above the
+/// inbox so a timeout cannot be starved by inbox traffic. Each fire carries the
+/// local id it was armed under so the loop can confirm it is still the live one
+/// (a fire that a reset/cancel beat onto the channel is discarded).
+enum Sys {
+    /// The state-timeout fired, carrying the local id it was armed under so the
+    /// loop can drop a stale fire (one a reset/cancel beat onto the channel).
+    StateTimeout(u64),
+    /// A named timeout fired, carrying its name and the local id it was armed
+    /// under.
+    Timeout(&'static str, u64),
+}
+
+// ---------------------------------------------------------------------------
+// Timers — shared timer bookkeeping
+// ---------------------------------------------------------------------------
+
+/// Per-machine timer bookkeeping, shared between the loop and every [`Cx`]
+/// borrow (a machine is single-threaded — handler calls and the loop never run
+/// concurrently — so this `Mutex` is always uncontended; it is `Arc<Mutex>`
+/// rather than `Rc<RefCell>` only because `Machine: Send` forces it).
+///
+/// Each arming mints a fresh **local id** carried in the fire payload and
+/// recorded here alongside the substrate id used to cancel. On fire the loop
+/// compares the payload's local id against the recorded one: a fire whose id no
+/// longer matches — a reset/cancel armed a newer one or removed the entry after
+/// this fire already escaped onto the channel — is stale and dropped. The local
+/// id is minted up front (unlike the substrate id, which only exists once armed),
+/// so it can ride the payload with no chicken-and-egg.
+struct Timers {
+    /// Monotonic minter for local ids, kept distinct from substrate ids (the
+    /// latter are what we hand to [`cancel_timer`]).
+    next_local: u64,
+    /// The currently-armed state-timeout's `(local id, substrate id)`. Cancelled
+    /// and cleared on every real state change (auto-reset), replaced on re-arm.
+    state: Option<(u64, TimerId)>,
+    /// Live named timeouts: name → `(local id, substrate id)`. Survives state
+    /// changes; an entry lives until it fires or is cancelled.
+    named: HashMap<&'static str, (u64, TimerId)>,
+}
+
+impl Timers {
+    fn new() -> Self {
+        Timers { next_local: 0, state: None, named: HashMap::new() }
+    }
+
+    fn mint(&mut self) -> u64 {
+        let id = self.next_local;
+        self.next_local = self.next_local.wrapping_add(1);
+        id
+    }
+}
+
 // ---------------------------------------------------------------------------
 // Cx — the per-handler context
 // ---------------------------------------------------------------------------
 
 /// The context handle injected into [`Machine::on_start`] and
-/// [`Machine::handle`]. Non-state outcomes (postpone, timeout arming) live here
-/// as method calls rather than keywords.
+/// [`Machine::handle`]. Non-state outcomes — timeout arming, and later
+/// postpone — live here as method calls rather than keywords. It lives only on
+/// the actor's own stack and is never sent.
 ///
-/// For now it carries only the [`on_unhandled`](Self::on_unhandled) hook;
-/// `cx.state_timeout(d)` / `cx.timeout(name, d)` and `cx.postpone()` will attach
-/// here when implemented. It lives only on the actor's own
-/// stack and is never sent.
+/// Timeouts armed here land on the loop's system channel through `sys_tx` and
+/// are tracked in the shared `reg` so reset/cancel can find the pending one.
 pub struct Cx<Ev> {
+    sys_tx: Sender<Sys>,
+    reg: Arc<Mutex<Timers>>,
     _ev: PhantomData<fn() -> Ev>,
 }
 
 impl<Ev> Cx<Ev> {
-    fn new() -> Self {
-        Cx { _ev: PhantomData }
+    fn new(sys_tx: Sender<Sys>, reg: Arc<Mutex<Timers>>) -> Self {
+        Cx { sys_tx, reg, _ev: PhantomData }
     }
 
-    /// The default for an event no arm matched: **log-and-drop**. Overridable
-    /// hook wiring is a follow-on; for now an unmatched event is
-    /// silently dropped, as `gen_server` does with unexpected messages.
+    /// Arm the **state timeout**: fire a `state_timeout` event after `after` in
+    /// the current state. Auto-reset on any state change (the loop cancels and
+    /// clears it on every real transition), so it measures quiet time *within* a
+    /// state. Only one is ever pending — arming again cancels and replaces the
+    /// previous one.
+    pub fn state_timeout(&mut self, after: Duration) {
+        let mut reg = match self.reg.lock() {
+            Ok(g) => g,
+            Err(e) => panic!("smarm: gen_statem reg lock poisoned (core corrupt): {e}"),
+        };
+        if let Some((_, old_sub)) = reg.state.take() {
+            cancel_timer(old_sub);
+        }
+        let local = reg.mint();
+        let sub = send_after_to(after, self.sys_tx.clone(), Sys::StateTimeout(local));
+        reg.state = Some((local, sub));
+    }
+
+    /// Arm a **named generic timeout**: fire a `timeout(name)` event after
+    /// `after`. Survives state changes; several may be in flight keyed by
+    /// `name`. Arming the same `name` again cancels and replaces the pending one
+    /// for that name.
+    pub fn timeout(&mut self, name: &'static str, after: Duration) {
+        let mut reg = match self.reg.lock() {
+            Ok(g) => g,
+            Err(e) => panic!("smarm: gen_statem reg lock poisoned (core corrupt): {e}"),
+        };
+        if let Some((_, old_sub)) = reg.named.remove(name) {
+            cancel_timer(old_sub);
+        }
+        let local = reg.mint();
+        let sub = send_after_to(after, self.sys_tx.clone(), Sys::Timeout(name, local));
+        reg.named.insert(name, (local, sub));
+    }
+
+    /// Cancel the named timeout `name` if pending. Returns `true` if the cancel
+    /// beat the fire, `false` if it had already fired / was never armed.
+    pub fn cancel_timeout(&mut self, name: &'static str) -> bool {
+        let mut reg = match self.reg.lock() {
+            Ok(g) => g,
+            Err(e) => panic!("smarm: gen_statem reg lock poisoned (core corrupt): {e}"),
+        };
+        match reg.named.remove(name) {
+            Some((_, sub)) => cancel_timer(sub),
+            None => false,
+        }
+    }
+
+    /// Cancel the pending state-timeout if any. Returns `true` if the cancel
+    /// beat the fire. Rarely needed by hand (the loop auto-resets on
+    /// transition); exposed for a handler that wants to disarm within a state.
+    pub fn cancel_state_timeout(&mut self) -> bool {
+        let mut reg = match self.reg.lock() {
+            Ok(g) => g,
+            Err(e) => panic!("smarm: gen_statem reg lock poisoned (core corrupt): {e}"),
+        };
+        match reg.state.take() {
+            Some((_, sub)) => cancel_timer(sub),
+            None => false,
+        }
+    }
+
+    /// The default for an event no arm matched: **log-and-drop**, as
+    /// `gen_server` does with unexpected messages.
     pub fn on_unhandled(&mut self) {}
+
+    /// Cancel and clear the pending state-timeout. Called by the macro on every
+    /// real transition (the state-timeout is auto-reset across state changes);
+    /// not part of the authoring surface. A handler re-arms in the new state's
+    /// `enter` if it wants one.
+    #[doc(hidden)]
+    pub fn __reset_state_timeout(&mut self) {
+        let mut reg = match self.reg.lock() {
+            Ok(g) => g,
+            Err(e) => panic!("smarm: gen_statem reg lock poisoned (core corrupt): {e}"),
+        };
+        if let Some((_, sub)) = reg.state.take() {
+            cancel_timer(sub);
+        }
+    }
 }
 
 // ---------------------------------------------------------------------------
@@ -143,9 +344,8 @@ impl<Ev> Cx<Ev> {
 /// machine dies, closing the channel).
 ///
 /// Carrying the handle in the event — rather than the loop owning a reply slot
-/// — is what lets a later chunk **postpone a call**: the whole event, reply
-/// handle included, moves onto the postpone queue and is answered by a later
-/// state.
+/// — is what will let a **postponed call** work: the whole event, reply handle
+/// included, moves onto the postpone queue and is answered by a later state.
 pub struct Reply<T> {
     tx: Sender<T>,
 }
@@ -211,9 +411,7 @@ impl<M: Machine> GenStatemRef<M> {
     /// stack unwinds, closing the channel and waking the caller).
     ///
     /// `make` wraps the [`Reply`] all the way to `M::Ev` — typically
-    /// `|r| Ev::Call(MyCall::GetCount(r))`. (The deferred macro would generate a
-    /// thinner `call` that hides the `Ev::Call` wrap and takes the bare variant
-    /// constructor.)
+    /// `|r| Ev::Call(MyCall::GetCount(r))`.
     pub fn call<T, F>(&self, make: F) -> Result<T, CallError>
     where
         T: Send + 'static,
@@ -241,18 +439,84 @@ pub fn spawn<M: Machine>(machine: M) -> GenStatemRef<M> {
     GenStatemRef { tx, pid: handle.pid() }
 }
 
-/// The machine actor body: `on_start`, then one `handle` per inbox event until
-/// the inbox closes (all refs dropped → graceful shutdown). Chunk 1 parks on
-/// the inbox alone — no system arm, no timers — the analogue of `gen_server`'s
-/// plain-inbox park.
+/// The machine actor body: `on_start`, then one `handle` per event until the
+/// inbox closes (all refs dropped → graceful shutdown).
+///
+/// Two intake sources are selected each iteration with the **timer arm above
+/// the inbox**, so a timeout fire is never starved by inbox traffic: `sys_rx`
+/// carries timer fires armed through `cx`, `rx` is the user inbox. A fire is
+/// turned into the matching internal event (`state_timeout` / `timeout(name)`)
+/// and run through the same `handle` dispatch as an inbox event — the
+/// gen_statem model, where timeouts surface as ordinary events.
+///
+/// The loop owns the **postpone queue**: a `handle` that defers its event hands
+/// it back ([`Step::Postponed`]) for the queue; a `handle` that transitions
+/// ([`Step::Transitioned`]) triggers a [`replay`] of the queue in the new
+/// state, ahead of the next intake.
 fn statem_loop<M: Machine>(rx: Receiver<M::Ev>, mut machine: M) {
-    let mut cx = Cx::new();
+    let (sys_tx, sys_rx) = channel::<Sys>();
+    let reg = Arc::new(Mutex::new(Timers::new()));
+    // The loop owns `cx` (and through it a `sys_tx` clone) for its whole life,
+    // so the sys arm never closes from under us — no auto-close dance needed.
+    let mut cx = Cx::new(sys_tx, reg.clone());
+    // Events deferred by `postpone` rows, replayed FIFO on the next transition.
+    let mut postpone: VecDeque<M::Ev> = VecDeque::new();
     machine.on_start(&mut cx);
     loop {
-        match rx.recv() {
-            Ok(ev) => machine.handle(ev, &mut cx),
-            // All StatemRefs dropped → inbox closed → shutdown.
-            Err(_) => break,
+        // Timer arm first: a ready fire is taken in preference to the inbox.
+        let i = select(&[&sys_rx, &rx]);
+        if i == 0 {
+            match sys_rx.try_recv() {
+                Ok(Some(fire)) => {
+                    // Confirm the fire is still the live one before dispatching:
+                    // a reset/cancel may have replaced/removed it after it
+                    // escaped onto the channel. A stale fire is dropped.
+                    let ev = match fire {
+                        Sys::StateTimeout(local) => {
+                            let mut t = match reg.lock() {
+                                Ok(g) => g,
+                                Err(e) => panic!("smarm: gen_statem reg lock poisoned (core corrupt): {e}"),
+                            };
+                            match t.state {
+                                Some((live, _)) if live == local => {
+                                    t.state = None; // retire: it has now fired
+                                    Some(M::state_timeout_ev())
+                                }
+                                _ => None,
+                            }
+                        }
+                        Sys::Timeout(name, local) => {
+                            let mut t = match reg.lock() {
+                                Ok(g) => g,
+                                Err(e) => panic!("smarm: gen_statem reg lock poisoned (core corrupt): {e}"),
+                            };
+                            match t.named.get(name) {
+                                Some(&(live, _)) if live == local => {
+                                    t.named.remove(name); // retire on fire
+                                    Some(M::timeout_ev(name))
+                                }
+                                _ => None,
+                            }
+                        }
+                    };
+                    if let Some(ev) = ev {
+                        dispatch(&mut machine, &mut cx, &mut postpone, ev);
+                    }
+                }
+                // Single-receiver: nothing can drain the arm between select's
+                // ready and our try_recv.
+                Ok(None) => debug_assert!(false, "ready system arm was empty"),
+                // The loop holds a sys_tx for its whole life, so this is
+                // unreachable; fall through defensively.
+                Err(_) => {}
+            }
+        } else {
+            match rx.try_recv() {
+                Ok(Some(ev)) => dispatch(&mut machine, &mut cx, &mut postpone, ev),
+                Ok(None) => debug_assert!(false, "ready inbox was empty"),
+                // All GenStatemRefs dropped → inbox closed → shutdown.
+                Err(_) => break,
+            }
         }
         // Observation point so a machine fed a hot inbox stays preemptible and
         // cancellable.
@@ -260,15 +524,60 @@ fn statem_loop<M: Machine>(rx: Receiver<M::Ev>, mut machine: M) {
     }
 }
 
+/// Run one event through `handle` and act on its [`Step`]: stash a deferred
+/// event on the postpone queue, or — on a real transition — [`replay`] the
+/// queue in the new state. A stay/unmatched event needs nothing further.
+fn dispatch<M: Machine>(
+    machine: &mut M,
+    cx: &mut Cx<M::Ev>,
+    postpone: &mut VecDeque<M::Ev>,
+    ev: M::Ev,
+) {
+    match machine.handle(ev, cx) {
+        Step::Postponed(ev) => postpone.push_back(ev),
+        Step::Stayed => {}
+        Step::Transitioned => replay(machine, cx, postpone),
+    }
+}
+
+/// Replay deferred events after a real transition: each goes back through
+/// `handle` in FIFO order, in the now-current state. An event that postpones
+/// again re-queues (to wait for the *next* transition); one that transitions
+/// re-arms the replay, so a later state can in turn drain what is still pending.
+/// Subsequent events in a batch already see the post-transition state, since
+/// `handle` reads the live state cell — the outer loop only re-runs to give
+/// re-queued events another pass once a transition has occurred within a batch.
+fn replay<M: Machine>(machine: &mut M, cx: &mut Cx<M::Ev>, postpone: &mut VecDeque<M::Ev>) {
+    loop {
+        if postpone.is_empty() {
+            return;
+        }
+        // Take the current backlog; anything deferred during this pass lands in
+        // the now-empty queue and is only retried if this pass also transitioned.
+        let batch = std::mem::take(postpone);
+        let mut transitioned = false;
+        for ev in batch {
+            match machine.handle(ev, cx) {
+                Step::Postponed(ev) => postpone.push_back(ev),
+                Step::Stayed => {}
+                Step::Transitioned => transitioned = true,
+            }
+        }
+        if !transitioned {
+            return;
+        }
+    }
+}
+
 // ---------------------------------------------------------------------------
-// gen_statem! — the authoring macro (fused total-match variant)
+// gen_statem! — the authoring macro (total-match dispatch)
 // ---------------------------------------------------------------------------
 
 /// Assemble a complete [`Machine`] from hand-written types, a glanceable
 /// transition table, and free handler functions.
 ///
-/// This is the **fused** authoring surface (see `examples/statem_fused.rs` for
-/// the same machine written out by hand). You keep ownership of every type that
+/// This is the authoring surface (see `examples/gen_statem_expanded.rs` for the
+/// same machine written out by hand). You keep ownership of every type that
 /// carries meaning — the state enum, the data struct, the `cast`/`call` enums,
 /// and any per-row successor enums — and the macro emits only the mechanical
 /// scaffolding: the unified event enum, the machine struct and its private
@@ -308,12 +617,12 @@ fn statem_loop<M: Machine>(rx: Receiver<M::Ev>, mut machine: M) {
 ///    machine lives in the same crate as this macro, and is **silently dropped
 ///    when you call `gen_statem!` from a downstream crate** — neither a crate
 ///    `#![deny]` nor `#![forbid]` overrides the suppression. This is the one
-///    guarantee a `macro_rules!` cannot carry across the crate boundary; the
-///    RFC's proc-macro could (it would stamp the arms with call-site spans).
+///    guarantee a `macro_rules!` cannot carry across the crate boundary; a
+///    proc-macro could (it would stamp the arms with call-site spans).
 ///    Guard against it in review, or with an in-crate test of the machine.
 ///
 /// There is deliberately **no separate `transitions { … }` adjacency block**:
-/// in this variant the `match` *is* the table and the successor enums *are* the
+/// the `match` *is* the table and the successor enums *are* the
 /// per-state target sets, so a second listing would be redundant and
 /// un-cross-checkable without a proc-macro. The graph is read off the `on`
 /// blocks and the successor enums at the top of the file.
@@ -323,14 +632,16 @@ fn statem_loop<M: Machine>(rx: Receiver<M::Ev>, mut machine: M) {
 /// ```ignore
 /// // ── your types (the macro never generates or inspects these) ───────────
 /// #[derive(Clone, Copy, PartialEq, Eq, Debug)]
-/// enum Switch { Off, On }
-/// struct Counts { flips: u32, enters: u32 }
-/// enum SwitchCast { Flip }
-/// enum SwitchCall { GetCount(Reply<u32>) }
+/// enum Door { Open, Closed, Locked }
+/// struct Data { enters: u32 }
+/// enum DoorCast { Push, Pull, Lock, Unlock(u32) }
+/// enum DoorCall { GetState(Reply<Door>) }
 ///
 /// gen_statem! {
-///     machine: SwitchSm { state: Switch, data: Counts };
-///     event:   Ev { cast: SwitchCast, call: SwitchCall };
+///     machine: DoorSm { state: Door, data: Data };
+///     // The event clause folds three user enums together. `info` is for
+///     // out-of-band messages; use `()` if you have none.
+///     event:   Ev { cast: DoorCast, call: DoorCall, info: () };
 ///
 ///     // Name the bindings your handler bodies use. A declarative macro can't
 ///     // hand you its own `self`/`cx` (hygiene), so you choose the identifiers
@@ -339,26 +650,51 @@ fn statem_loop<M: Machine>(rx: Receiver<M::Ev>, mut machine: M) {
 ///     context(data, prev, cx);
 ///
 ///     // `enter` runs on entry to a state; side effects only, returns ().
-///     // Match the state tag (or `_`); the arm body is statements.
+///     // Match the state tag (or `_`); the arm body is statements. Arm any
+///     // state-timeout here (it is auto-reset on the way into a new state).
 ///     enter {
+///         Door::Open => {
+///             data.enters += 1;
+///             cx.state_timeout(std::time::Duration::from_secs(30)); // auto-close
+///         }
 ///         _ => data.enters += 1,
 ///     }
 ///
 ///     // The transition table. Group rows by current state with `on <pat>`.
-///     // A row is:  cast|call  <event-pattern>  [if <guard>]  =>  <tail> ,
+///     // A row is:  <kind> <event-pattern> [if <guard>]  =>  <tail> ,
+///     // where <kind> is one of `cast`, `call`, `info`, `state_timeout`
+///     // (no pattern — it is a unit event), or `timeout <name-pattern>`,
 ///     // and the tail is one of:
-///     //   * a state tag            `Switch::On`        (transition, or "stay"
-///     //                                                  if it equals current)
-///     //   * a block ending in one  `{ data.flips += 1; Switch::On }`
-///     //   * a successor-enum value `unlock(key)`       (branching row)
-///     //   * the keyword            `unhandled`         (explicit refusal)
-///     on Switch::Off => {
-///         cast SwitchCast::Flip            => { data.flips += 1; Switch::On },
-///         call SwitchCall::GetCount(r)     => { r.reply(data.flips); prev },
+///     //   * a state tag            `Door::Closed`     (transition, or "stay"
+///     //                                                 if it equals current)
+///     //   * a block ending in one  `{ data.enters += 1; Door::Closed }`
+///     //   * a successor-enum value `on_unlock(key)`   (branching row)
+///     //   * the keyword            `unhandled`        (explicit refusal)
+///     //   * the keyword            `postpone`         (defer until next
+///     //                                                transition; cast/call/
+///     //                                                info only)
+///     on Door::Open => {
+///         cast DoorCast::Push                       => Door::Closed,
+///         // An armed state-timeout surfaces as an ordinary event:
+///         state_timeout                             => Door::Closed,
+///         cast DoorCast::Pull | DoorCast::Lock | DoorCast::Unlock(_) => unhandled,
 ///     }
-///     on Switch::On => {
-///         cast SwitchCast::Flip            => Switch::Off,
-///         call SwitchCall::GetCount(r)     => { r.reply(data.flips); prev },
+///     on Door::Closed => {
+///         cast DoorCast::Pull                       => Door::Open,
+///         cast DoorCast::Lock                       => Door::Locked,
+///         cast DoorCast::Push | DoorCast::Unlock(_) => unhandled,
+///         // No state-timeout armed here, so refuse it.
+///         state_timeout                             => unhandled,
+///     }
+///     on Door::Locked => {
+///         cast DoorCast::Unlock(key)                => on_unlock(key), // -> successor enum
+///         cast DoorCast::Push | DoorCast::Pull | DoorCast::Lock => unhandled,
+///         state_timeout                             => unhandled,
+///     }
+///
+///     // state-independent queries (reply, then stay via `prev`)
+///     on _ => {
+///         call DoorCall::GetState(r) => { r.reply(prev); prev },
 ///     }
 /// }
 /// ```
@@ -368,30 +704,53 @@ fn statem_loop<M: Machine>(rx: Receiver<M::Ev>, mut machine: M) {
 /// * **Every row ends in a comma** (block-bodied ones too) and every `on` block
 ///   is `on <state-pat> => { … }`. These two are macro-grammar requirements, not
 ///   style: a declarative matcher can't otherwise tell where a row's tail ends.
-/// * **Qualify event patterns** (`SwitchCast::Flip`) and **state tags**
-///   (`Switch::On`). A declarative macro can't prepend the enum name inside an
-///   opaque pattern fragment, so the `cast`/`call` keyword only selects the
-///   event wrapper — it does not qualify for you. The keyword also reads as a
-///   sync/async marker at a glance.
+/// * **Qualify event patterns** (`DoorCast::Push`) and **state tags**
+///   (`Door::Closed`). A declarative macro can't prepend the enum name inside an
+///   opaque pattern fragment, so the row keyword only selects the event wrapper
+///   — it does not qualify for you. `cast`/`call` also read as a sync/async
+///   marker at a glance.
+/// * **Timeouts surface as events.** `state_timeout` (a unit event) and
+///   `timeout <name>` (matching the `&'static str` a generic timeout was armed
+///   under) are matched in `on` rows just like casts and calls — arm them via
+///   `cx`, handle the fire here. Because a `timeout` name is an `&str`, a row
+///   that matches specific names needs a `timeout _ => …` fallback to stay
+///   exhaustive.
+/// * **`info` defaults to a silent drop.** An out-of-band `info` you do not
+///   match anywhere is dropped (the `gen_server` default), so a machine that
+///   ignores info writes no `info` rows at all. State-timeouts and named
+///   timeouts have **no** such default: a state that can see one must handle it
+///   (or `unhandled` it) or the match is non-exhaustive.
 /// * **Stay** = return the current tag. The `prev` you named in `context` is
 ///   bound to the pre-handler state for exactly this — handy in any-state
 ///   (`on _`) rows where there is no single literal tag to write.
 /// * **`data` and `cx`** (the names you chose) are in scope in every body:
-///   mutate `data`, reply through a `Reply` bound in the pattern, and (chunks
-///   2–3) arm timeouts or postpone via `cx`. You never write `self`.
+///   mutate `data`, reply through a `Reply` bound in the pattern, and arm
+///   timeouts via `cx`. You never write `self`.
 /// * **Guards** are plain match guards. A guarded arm does not count toward
 ///   exhaustiveness, so pair it with an unguarded fallback or you'll (correctly)
 ///   trip `E0004`.
 /// * **Branching rows** return a successor enum that `impl`s `From<_>` for the
 ///   state type; the macro supplies the outer `.into()`.
+/// * **`postpone`** defers the current event untouched, to be replayed after the
+///   next *real transition* (a stay does not trigger replay). It is available
+///   for `cast`, `call`, and `info` rows — not the timeout events. The deferred
+///   event keeps any `Reply` it carries, so a postponed `call` is answered by
+///   whichever later state handles the replay. The body runs *no* code (the
+///   event is untouched), so a `postpone` row is just `cast Foo(_) => postpone,`;
+///   prefer a non-binding pattern, and if you guard it, the guard must not depend
+///   on the event's payload (it is evaluated in a borrow pre-pass). On a
+///   transition the queue drains FIFO, ahead of further inbox/timer events; a
+///   replayed event may postpone again (it re-queues for the next transition).
 ///
 /// # What it emits
 ///
-/// `enum $Ev { Cast($Cast), Call($Call) }`, `struct $Sm { state, data }`,
-/// `$Sm::start(init, data) -> GenStatemRef<$Sm>`, the `Machine` impl (`on_start`
-/// running the initial `enter`; `handle` = the dispatch match + the
-/// stay/transition/unhandled apply-tail, the cell's sole writer), and the
-/// `enter` dispatch. Chunk 1: real time, no timers, no postpone.
+/// The unified `enum $Ev` (the `Cast`/`Call`/`Info` wrappers plus the internal
+/// `StateTimeout` / `Timeout` events), `struct $Sm { state, data }`,
+/// `$Sm::start(init, data) -> GenStatemRef<$Sm>`, and the `Machine` impl:
+/// `on_start` runs the initial `enter`; `handle` is the dispatch match plus the
+/// stay/transition/unhandled apply-tail (the cell's sole writer, which also
+/// auto-resets the state-timeout on every real transition); and the `enter`
+/// dispatch.
 ///
 /// # Limitation
 ///
@@ -403,16 +762,23 @@ macro_rules! gen_statem {
     // ===== public entry =====================================================
     (
         machine: $sm:ident { state: $State:ty, data: $Data:ty } ;
-        event:   $Ev:ident { cast: $Cast:ty, call: $Call:ty } ;
+        event:   $Ev:ident { cast: $Cast:ty, call: $Call:ty, info: $Info:ty } ;
         context ( $data:ident , $cur:ident , $cx:ident ) ;
         enter { $( $est:pat => $ebody:expr ),+ $(,)? }
         $( on $st:pat => { $($rows:tt)* } )+
     ) => {
-        /// Unified inbox payload: the user's `cast`/`call` enums folded together
-        /// (chunks 2–3 add the runtime's internal timeout variants here).
+        /// Unified inbox payload: the user's `cast`/`call`/`info` enums folded
+        /// together with the runtime's internal timeout events.
         enum $Ev {
             Cast($Cast),
             Call($Call),
+            /// Out-of-band message, matched in `info` rows. An unmatched info is
+            /// silently dropped (the `gen_server` default).
+            Info($Info),
+            /// The state-timeout fired (matched in `state_timeout` rows).
+            StateTimeout,
+            /// A named timeout fired (matched in `timeout <pat>` rows).
+            Timeout(&'static str),
         }
 
         struct $sm {
@@ -438,6 +804,14 @@ macro_rules! gen_statem {
         impl $crate::gen_statem::Machine for $sm {
             type Ev = $Ev;
 
+            fn state_timeout_ev() -> $Ev {
+                $Ev::StateTimeout
+            }
+
+            fn timeout_ev(name: &'static str) -> $Ev {
+                $Ev::Timeout(name)
+            }
+
             fn on_start(&mut self, $cx: &mut $crate::gen_statem::Cx<$Ev>) {
                 let s = self.state;
                 self.enter(s, $cx);
@@ -446,79 +820,205 @@ macro_rules! gen_statem {
             #[allow(unused_variables)]
             #[deny(unreachable_patterns)] // conflicting rows must fail even though
                                           // this match is external-macro-expanded
-            fn handle(&mut self, ev: $Ev, $cx: &mut $crate::gen_statem::Cx<$Ev>) {
+            fn handle(&mut self, ev: $Ev, $cx: &mut $crate::gen_statem::Cx<$Ev>)
+                -> $crate::gen_statem::Step<$Ev>
+            {
                 // Caller-named bindings (shared call-site hygiene, so row bodies
                 // can see them): `$cur` = current state tag, `$data` = &mut Data.
                 let $cur = self.state;
                 let $data = &mut self.data;
+                // @arms emits a block: a postpone pre-pass that `return`s
+                // `Step::Postponed(ev)` for a deferred event (handing it back
+                // untouched), then the consuming `match (state, event)` whose
+                // value is this `Resolution`.
                 let next: $crate::gen_statem::Resolution<$State> =
-                    $crate::gen_statem!(@arms ($Ev) ($cur, ev) [ ]
+                    $crate::gen_statem!(@arms ($Ev) ($cur, ev) [ ] [ ]
                         $( on $st => { $($rows)* } )+);
                 match next {
-                    $crate::gen_statem::Resolution::To(s) if s == $cur => {}
+                    $crate::gen_statem::Resolution::To(s) if s == $cur => {
+                        $crate::gen_statem::Step::Stayed
+                    }
                     $crate::gen_statem::Resolution::To(s) => {
                         self.state = s; // <- sole writer of the state cell
+                        // A real transition auto-resets the state-timeout: the
+                        // new state re-arms in its `enter` if it wants one.
+                        $cx.__reset_state_timeout();
                         self.enter(s, $cx);
+                        $crate::gen_statem::Step::Transitioned
                     }
-                    $crate::gen_statem::Resolution::Postpone => {
-                        unreachable!("postpone is not generated yet")
+                    $crate::gen_statem::Resolution::Unhandled => {
+                        $cx.on_unhandled();
+                        $crate::gen_statem::Step::Stayed
                     }
-                    $crate::gen_statem::Resolution::Unhandled => $cx.on_unhandled(),
                 }
             }
         }
     };
 
-    // ===== @arms: build the dispatch match ==================================
-    // No more on-blocks: emit the (total, catch-all-free) match.
-    (@arms ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ]) => {
-        match ($ss, $se) { $($arms)* }
+    // ===== @arms / @rows: build the two-phase dispatch ======================
+    // Two accumulators are threaded: `[ $($arms)* ]` is the phase-2 consuming
+    // match (by value); `[ $($post)* ]` is the phase-1 postpone router (by ref).
+    // A normal row feeds only phase-2; a `postpone` row feeds both — a `=> true`
+    // router and a `=> unreachable!` phase-2 filler that keeps the consuming
+    // match total.
+    //
+    // Terminal (no on-blocks left): emit the block the `handle` body assigns to
+    // `next`. Phase 1 routes a deferred event back out as `Step::Postponed`;
+    // phase 2 is the catch-all-free consuming match (the one macro-injected arm
+    // is the `Info` silent-drop — last and broadest, so per-state `info` rows
+    // stay reachable; cast/call/timeouts get no fallback, so a forgotten pair is
+    // still E0004).
+    (@arms ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ]) => {
+        {
+            // Phase 1 — postpone routing (borrow-only). A guard on a postpone
+            // row runs here, against by-ref bindings, so it must not depend on
+            // the event's payload.
+            let __defer = match ($ss, &$se) {
+                $($post)*
+                _ => false,
+            };
+            if __defer {
+                return $crate::gen_statem::Step::Postponed($se);
+            }
+            // Phase 2 — the consuming dispatch. Each postpone pair reappears here
+            // as `unreachable!` so the match stays total; phase 1 already
+            // returned for it.
+            match ($ss, $se) {
+                $($arms)*
+                (_, $Ev::Info(_)) => $crate::gen_statem::Resolution::Unhandled,
+            }
+        }
     };
     // Open an on-block: remember its state pat, drain its rows, then continue.
-    (@arms ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ]
+    (@arms ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ]
         on $st:pat => { $($rows:tt)* } $($more:tt)*
     ) => {
-        $crate::gen_statem!(@rows ($Ev) ($ss, $se) [ $($arms)* ] ($st)
+        $crate::gen_statem!(@rows ($Ev) ($ss, $se) [ $($arms)* ] [ $($post)* ] ($st)
             { $($rows)* } { $($more)* })
     };
 
-    // ===== @rows: drain one on-block's rows, threading the global acc ========
+    // ===== @rows: drain one on-block's rows, threading both accs =============
     // cast, explicit refusal
-    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] ($st:pat)
+    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ] ($st:pat)
         { cast $ev:pat $(if $g:expr)? => unhandled , $($rows:tt)* } { $($more:tt)* }
     ) => {
         $crate::gen_statem!(@rows ($Ev) ($ss, $se)
             [ $($arms)* ($st, $Ev::Cast($ev)) $(if $g)? => $crate::gen_statem::Resolution::Unhandled, ]
+            [ $($post)* ]
+            ($st) { $($rows)* } { $($more)* })
+    };
+    // cast, postpone (defer the event; the replay in a later state handles it)
+    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ] ($st:pat)
+        { cast $ev:pat $(if $g:expr)? => postpone , $($rows:tt)* } { $($more:tt)* }
+    ) => {
+        $crate::gen_statem!(@rows ($Ev) ($ss, $se)
+            [ $($arms)* ($st, $Ev::Cast($ev)) $(if $g)? => unreachable!("postponed event is replayed, not dispatched here"), ]
+            [ $($post)* ($st, $Ev::Cast($ev)) $(if $g)? => true, ]
             ($st) { $($rows)* } { $($more)* })
     };
     // cast, transition / stay / branch
-    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] ($st:pat)
+    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ] ($st:pat)
         { cast $ev:pat $(if $g:expr)? => $tail:expr , $($rows:tt)* } { $($more:tt)* }
     ) => {
         $crate::gen_statem!(@rows ($Ev) ($ss, $se)
             [ $($arms)* ($st, $Ev::Cast($ev)) $(if $g)? => $crate::gen_statem::Resolution::To($tail.into()), ]
+            [ $($post)* ]
             ($st) { $($rows)* } { $($more)* })
     };
     // call, explicit refusal
-    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] ($st:pat)
+    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ] ($st:pat)
         { call $ev:pat $(if $g:expr)? => unhandled , $($rows:tt)* } { $($more:tt)* }
     ) => {
         $crate::gen_statem!(@rows ($Ev) ($ss, $se)
             [ $($arms)* ($st, $Ev::Call($ev)) $(if $g)? => $crate::gen_statem::Resolution::Unhandled, ]
+            [ $($post)* ]
+            ($st) { $($rows)* } { $($more)* })
+    };
+    // call, postpone (the Reply rides inside the event onto the queue)
+    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ] ($st:pat)
+        { call $ev:pat $(if $g:expr)? => postpone , $($rows:tt)* } { $($more:tt)* }
+    ) => {
+        $crate::gen_statem!(@rows ($Ev) ($ss, $se)
+            [ $($arms)* ($st, $Ev::Call($ev)) $(if $g)? => unreachable!("postponed event is replayed, not dispatched here"), ]
+            [ $($post)* ($st, $Ev::Call($ev)) $(if $g)? => true, ]
             ($st) { $($rows)* } { $($more)* })
     };
     // call, transition / stay / branch
-    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] ($st:pat)
+    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ] ($st:pat)
         { call $ev:pat $(if $g:expr)? => $tail:expr , $($rows:tt)* } { $($more:tt)* }
     ) => {
         $crate::gen_statem!(@rows ($Ev) ($ss, $se)
             [ $($arms)* ($st, $Ev::Call($ev)) $(if $g)? => $crate::gen_statem::Resolution::To($tail.into()), ]
+            [ $($post)* ]
+            ($st) { $($rows)* } { $($more)* })
+    };
+    // info, explicit refusal
+    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ] ($st:pat)
+        { info $ev:pat $(if $g:expr)? => unhandled , $($rows:tt)* } { $($more:tt)* }
+    ) => {
+        $crate::gen_statem!(@rows ($Ev) ($ss, $se)
+            [ $($arms)* ($st, $Ev::Info($ev)) $(if $g)? => $crate::gen_statem::Resolution::Unhandled, ]
+            [ $($post)* ]
+            ($st) { $($rows)* } { $($more)* })
+    };
+    // info, postpone
+    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ] ($st:pat)
+        { info $ev:pat $(if $g:expr)? => postpone , $($rows:tt)* } { $($more:tt)* }
+    ) => {
+        $crate::gen_statem!(@rows ($Ev) ($ss, $se)
+            [ $($arms)* ($st, $Ev::Info($ev)) $(if $g)? => unreachable!("postponed event is replayed, not dispatched here"), ]
+            [ $($post)* ($st, $Ev::Info($ev)) $(if $g)? => true, ]
+            ($st) { $($rows)* } { $($more)* })
+    };
+    // info, transition / stay / branch
+    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ] ($st:pat)
+        { info $ev:pat $(if $g:expr)? => $tail:expr , $($rows:tt)* } { $($more:tt)* }
+    ) => {
+        $crate::gen_statem!(@rows ($Ev) ($ss, $se)
+            [ $($arms)* ($st, $Ev::Info($ev)) $(if $g)? => $crate::gen_statem::Resolution::To($tail.into()), ]
+            [ $($post)* ]
+            ($st) { $($rows)* } { $($more)* })
+    };
+    // state_timeout, explicit refusal (unit event — no pattern; not postponable)
+    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ] ($st:pat)
+        { state_timeout $(if $g:expr)? => unhandled , $($rows:tt)* } { $($more:tt)* }
+    ) => {
+        $crate::gen_statem!(@rows ($Ev) ($ss, $se)
+            [ $($arms)* ($st, $Ev::StateTimeout) $(if $g)? => $crate::gen_statem::Resolution::Unhandled, ]
+            [ $($post)* ]
+            ($st) { $($rows)* } { $($more)* })
+    };
+    // state_timeout, transition / stay / branch
+    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ] ($st:pat)
+        { state_timeout $(if $g:expr)? => $tail:expr , $($rows:tt)* } { $($more:tt)* }
+    ) => {
+        $crate::gen_statem!(@rows ($Ev) ($ss, $se)
+            [ $($arms)* ($st, $Ev::StateTimeout) $(if $g)? => $crate::gen_statem::Resolution::To($tail.into()), ]
+            [ $($post)* ]
+            ($st) { $($rows)* } { $($more)* })
+    };
+    // timeout, explicit refusal (pattern matches the name; not postponable)
+    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ] ($st:pat)
+        { timeout $ev:pat $(if $g:expr)? => unhandled , $($rows:tt)* } { $($more:tt)* }
+    ) => {
+        $crate::gen_statem!(@rows ($Ev) ($ss, $se)
+            [ $($arms)* ($st, $Ev::Timeout($ev)) $(if $g)? => $crate::gen_statem::Resolution::Unhandled, ]
+            [ $($post)* ]
+            ($st) { $($rows)* } { $($more)* })
+    };
+    // timeout, transition / stay / branch
+    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ] ($st:pat)
+        { timeout $ev:pat $(if $g:expr)? => $tail:expr , $($rows:tt)* } { $($more:tt)* }
+    ) => {
+        $crate::gen_statem!(@rows ($Ev) ($ss, $se)
+            [ $($arms)* ($st, $Ev::Timeout($ev)) $(if $g)? => $crate::gen_statem::Resolution::To($tail.into()), ]
+            [ $($post)* ]
             ($st) { $($rows)* } { $($more)* })
     };
     // this block is drained: hand the remaining on-blocks back to @arms
-    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] ($st:pat)
+    (@rows ($Ev:ident) ($ss:expr, $se:expr) [ $($arms:tt)* ] [ $($post:tt)* ] ($st:pat)
         { } { $($more:tt)* }
     ) => {
-        $crate::gen_statem!(@arms ($Ev) ($ss, $se) [ $($arms)* ] $($more)*)
+        $crate::gen_statem!(@arms ($Ev) ($ss, $se) [ $($arms)* ] [ $($post)* ] $($more)*)
     };
 }

src/introspect.rs

@@ -1,26 +1,85 @@
-//! RFC 016 — runtime introspection (Chunk 1: the read primitive).
+//! Inspect what is running right now: which actors exist, what state each one
+//! is in, and how they are related.
 //!
-//! A synchronous, internal read of the slab that returns *owned* data. This is
-//! the mechanism the whole RFC hangs off: tests, the future observer
-//! gen_server (Chunk 4), and a later control plane (RFC 003) are all consumers
-//! of [`snapshot`] / [`actor_info`], never of the runtime internals directly.
+//! This is the tool for questions like "is my server still alive", "how many
+//! actors are currently parked waiting on something", or "what does the spawn
+//! tree look like". It is meant for debugging, test assertions, a health check
+//! endpoint, or a monitoring dashboard: anywhere you want to look at the
+//! runtime from the outside without stopping it or coupling your code to its
+//! internals.
 //!
-//! ## Consistency (DECISION D2 — per-slot tearing, `ps` semantics)
+//! Three entry points, in order of scope:
 //!
-//! [`snapshot`] is point-in-time and mildly racy *across* actors: each slot's
-//! scheduling state is a lock-free word load, so an actor reported `Running`
-//! may already be `Parked`, and an actor can die mid-scan. This is the cheap,
-//! useful model (a coherent stop-the-world cut is expensive and rarely wanted).
-//! [`actor_info`] is coherent for the single actor it names.
+//! - [`snapshot`] returns every actor that currently exists, as a plain
+//!   owned `Vec`, so you can filter, count, or search it however you like.
+//! - [`actor_info`] returns a coherent view of exactly one actor, by pid.
+//!   Cheaper than filtering a whole snapshot down to one entry, and more
+//!   precise (see "Consistency" below).
+//! - [`tree`] returns the same actors as [`snapshot`], folded into a
+//!   parent/child forest that mirrors who spawned whom.
 //!
-//! ## Locking
+//! ```
+//! use smarm::{actor_info, channel, run, snapshot, spawn, ActorState};
 //!
-//! The lock order is **Leaf → Channel, at most one of each** (`raw_mutex.rs`);
-//! cold locks, the registry, and the free list are all Leaves, so we may never
-//! hold two at once. The read is therefore phased: first a single registry-leaf
-//! pass for names and mailbox depth (the per-channel length read is a Channel
-//! lock taken under that Leaf — legal), released before the slab scan takes any
-//! per-slot cold Leaf.
+//! run(|| {
+//!     let (ready_tx, ready_rx) = channel::<()>();
+//!     let (gate_tx, gate_rx) = channel::<()>();
+//!
+//!     let worker = spawn(move || {
+//!         ready_tx.send(()).unwrap();
+//!         gate_rx.recv().unwrap(); // blocks here until released
+//!     });
+//!     ready_rx.recv().unwrap();
+//!
+//!     // `snapshot` sees every actor, including this one and the worker.
+//!     let snap = snapshot();
+//!     assert!(snap.actors.len() >= 2);
+//!
+//!     // `actor_info` gives a coherent view of just the worker. It is
+//!     // blocked on the gate channel, so it must be Parked.
+//!     let pid = worker.pid();
+//!     let info = actor_info(pid).expect("worker is still alive");
+//!     assert_eq!(info.state, ActorState::Parked);
+//!
+//!     gate_tx.send(()).unwrap();
+//!     worker.join().unwrap();
+//!
+//!     // Once joined, the pid no longer names a live actor.
+//!     assert!(actor_info(pid).is_none());
+//! });
+//! ```
+//!
+//! ## Consistency
+//!
+//! [`snapshot`] is not a single atomic pause-the-world freeze: it walks every
+//! actor's state one after another, so it is a series of independent,
+//! cheap, lock-free reads rather than one coherent moment in time. Between
+//! reading actor A and actor B, either one can change state, and an actor can
+//! even finish and disappear mid-scan. In practice this is exactly what you
+//! want: a coherent stop-the-world snapshot would mean pausing every actor in
+//! the runtime just to look at it, which is expensive and rarely necessary
+//! for a dashboard, a test assertion, or a debugging session.
+//!
+//! [`actor_info`], in contrast, is coherent for the one actor it names: all of
+//! its fields describe the same instant for that actor, because a single
+//! actor's data cannot tear the way a scan across many actors can.
+//!
+//! ## Implementation notes
+//!
+//! These details matter if you are working on smarm itself; they are not part
+//! of the public contract.
+//!
+//! The read never stops the scheduler and never holds a lock across the whole
+//! scan. Each actor's scheduling state is a single lock-free word load
+//! (hence the possible tearing described above). Reading the rest of an
+//! actor's cold data (its supervisor, monitors, links, and so on) takes a
+//! brief per-actor lock, just long enough to copy those fields out; nothing
+//! is held across actors. Locking follows the crate-wide rule that at most
+//! one "leaf" lock (a per-actor lock, the registry lock, or the free list
+//! lock) is held at a time, with no leaf lock held while acquiring another.
+//! The read is phased accordingly: first one pass over the registry to
+//! collect every actor's registered names and mailbox depth, released before
+//! the per-actor scan begins.
 
 use crate::pid::Pid;
 use crate::registry::MailboxInfo;
@@ -31,15 +90,28 @@ use crate::slot_state::{
 };
 use std::collections::HashMap;
 
-/// Snapshot wire-format version (DECISION D1). [`RuntimeSnapshot`] is treated as
-/// a stable type from day one: it becomes the observer protocol (Chunk 4) and
-/// crosses a version boundary the moment a remote observer attaches (RFC 011),
-/// so the version travels with the data from the start.
+/// The format version carried by every [`RuntimeSnapshot`] and
+/// [`RuntimeTree`], as [`RuntimeSnapshot::format_version`] /
+/// [`RuntimeTree::format_version`]. If you serialize a snapshot (for example
+/// to send it somewhere else, or to compare snapshots taken with different
+/// versions of smarm) check this field: a change in its value means the shape
+/// of [`ActorInfo`] or its neighbors has changed and old and new snapshots
+/// should not be assumed compatible. If you only ever read a snapshot
+/// in-process in the same version of smarm that produced it, you can ignore
+/// this field.
 pub const SNAPSHOT_FORMAT_VERSION: u16 = 1;
 
-/// Fine-grained scheduling state, mapped from the packed slot word with no new
-/// storage. `RunningNotified` collapses into `Notified` — a wake landed while
-/// the actor was on-CPU and it will re-queue when it yields.
+/// What an actor is doing right now, from the scheduler's point of view.
+///
+/// - `Queued`: runnable, waiting for a scheduler thread to pick it up.
+/// - `Running`: currently executing on a scheduler thread.
+/// - `Notified`: was running and got woken up (for example, a message
+///   arrived) before it had a chance to yield or park; it will be re-queued
+///   as soon as it does.
+/// - `Parked`: blocked, waiting on something such as a channel receive, a
+///   mutex, a timer, or an IO event.
+/// - `Done`: has finished (returned or panicked) but its slot has not been
+///   reclaimed for reuse yet, so it is still visible to introspection.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum ActorState {
     Queued,
@@ -49,8 +121,8 @@ pub enum ActorState {
     Done,
 }
 
-/// Classify a packed state word. `None` for a Vacant slot (skipped by the scan)
-/// — the only state that is not an actor.
+/// Classify a packed state word. `None` for a Vacant slot (skipped by the
+/// scan): a vacant slot holds no actor at all, live or done.
 fn classify(w: u64) -> Option<ActorState> {
     Some(match word_state(w) {
         ST_QUEUED => ActorState::Queued,
@@ -62,61 +134,76 @@ fn classify(w: u64) -> Option<ActorState> {
     })
 }
 
-/// Owned, point-in-time view of one actor — no borrows of runtime internals, so
-/// it is safe to hand to any consumer.
+/// An owned, self-contained view of one actor at (approximately) one moment.
+/// It borrows nothing from the runtime, so you can keep it, send it
+/// elsewhere, or print it long after the actor it describes has changed
+/// state or even exited.
 #[derive(Debug, Clone)]
 pub struct ActorInfo {
     pub pid: Pid,
-    /// Registered names, inverted from the registry (usually 0 or 1).
+    /// Names this actor is currently registered under (see the
+    /// [`registry`](crate::registry) module). Usually empty or one name;
+    /// an actor can have more if it registered several.
     pub names: Vec<&'static str>,
     pub state: ActorState,
-    /// Spawn-time parent edge (DECISION D9): `spawn_under` sets it to the
-    /// supervisor, plain `spawn` to the spawning actor — so it is parentage,
-    /// not necessarily a supervision relationship. `ROOT_PID` for the run's
-    /// root actor and for `Done` tombstones (whose `Actor` is already gone).
+    /// The actor that spawned this one: whoever called `spawn` or
+    /// `spawn_under` to create it. This is a parentage record, not
+    /// necessarily a supervision relationship: `spawn_under` records the
+    /// supervisor you asked for, while plain `spawn` records the spawning
+    /// actor itself, whether or not it supervises anything. It is the
+    /// runtime's root pid for the run's own root actor, and for a `Done`
+    /// actor whose bookkeeping has already been cleared.
     pub supervisor: Pid,
     pub trap_exit: bool,
     pub monitors: u32,
     pub links: u32,
     pub joiners: u32,
-    /// Queued messages summed over the actor's *published* channels (register /
-    /// install / spawn_addr / gen_server). 0 for an actor that holds only a
-    /// private `channel()` receiver — those are invisible to the registry.
+    /// Messages currently queued and not yet delivered, summed across every
+    /// channel this actor has published (via `register`, `install`,
+    /// `spawn_addr`, or starting a gen_server). This is 0 for an actor that
+    /// only holds a private, unpublished `channel()` receiver, since nothing
+    /// outside the actor can see that channel exists.
     pub mailbox_depth: u32,
-    /// Timeslice overruns tallied for this incarnation (RFC 016 Chunk 2): how
-    /// many times the actor was preempted for exceeding its slice. Resets on
-    /// restart (per-incarnation, D7).
+    /// How many times this actor has been preempted for running past its
+    /// scheduling timeslice. Counts only since the actor's current start (a
+    /// supervisor restart begins a fresh count).
     pub overruns: u64,
-    /// Messages this actor has received (dequeued) this incarnation (RFC 016
-    /// Chunk 2) — answers "is this actor a hotspot / draining slower than its
-    /// mailbox fills." Counts received, not sent (D4). Per-incarnation (D7).
+    /// How many messages this actor has received (taken off its inbox), since
+    /// its current start. Useful for spotting an actor whose mailbox is
+    /// filling up faster than it can drain it: compare this against
+    /// `mailbox_depth` over time.
     pub messages_received: u64,
-    /// Approximate on-CPU cycles this incarnation has consumed (RFC 016 Chunk 2)
-    /// — a reductions-like work metric for relative comparison. Always 0 unless
-    /// the `budget-accounting` feature is enabled (it costs an RDTSC per resume,
-    /// D6). Per-incarnation (D7).
+    /// Approximate CPU cycles this actor has spent running, since its current
+    /// start. A relative measure for comparing actors against each other, not
+    /// an absolute or wall-clock figure. Always 0 unless the crate's
+    /// `budget-accounting` feature is enabled, since measuring it costs a
+    /// timestamp read on every resume.
     pub budget_cycles: u64,
 }
 
-/// A whole-runtime snapshot. See the module docs for the D2 tearing model.
+/// A snapshot of every actor in the runtime at (approximately) one moment.
+/// See the module docs' "Consistency" section for what "approximately" means
+/// here.
 #[derive(Debug, Clone)]
 pub struct RuntimeSnapshot {
     pub format_version: u16,
     pub actors: Vec<ActorInfo>,
 }
 
-/// Snapshot every live (and `Done`-but-not-yet-reclaimed) actor on the slab.
-/// O(n) over the slot table, running with preemption disabled (like every
-/// runtime primitive) but holding no lock across the scan. Panics outside
-/// `Runtime::run()`; callable from actor code and the run thread.
+/// Every actor that currently exists: running, queued, parked, or finished
+/// but not yet cleaned up. Cheap and lock-free per actor; see the module
+/// docs for what "approximately one moment" means for the result as a whole.
+/// Panics if called outside [`run`](crate::run).
 pub fn snapshot() -> RuntimeSnapshot {
     with_runtime(|inner| {
-        // Phase A: one registry-leaf pass for names + mailbox depth, released
-        // before any cold leaf (no two Leaves at once).
+        // First pass: one registry lock to collect names + mailbox depth for
+        // every actor, released before touching any per-actor lock below.
         let mail = inner.registry.lock().introspect_map();
 
-        // Phase B: lock-free slab scan; per-slot cold leaf only to copy cold
-        // fields. Tearing across slots is intentional (D2).
+        // Second pass: walk the actor table. Each actor's scheduling state is
+        // a lock-free word load; only copying its other fields takes a brief
+        // per-actor lock. Tearing across actors is expected here (see the
+        // module docs' "Consistency" section).
         let mut actors = Vec::new();
         for (idx, slot) in inner.slots.iter().enumerate() {
             let idx = idx as u32;
@@ -128,8 +215,11 @@ pub fn snapshot() -> RuntimeSnapshot {
     })
 }
 
-/// Coherent view of a single actor, or `None` if the pid is stale, out of
-/// range, or names a Vacant slot.
+/// A coherent view of exactly one actor, or `None` if `pid` does not name a
+/// currently-live entry: it is stale (that actor has already exited and its
+/// slot was reused by another), out of range, or was never a real pid at
+/// all. Unlike [`snapshot`], every field of the result describes the same
+/// instant, since there is only one actor to read.
 pub fn actor_info(pid: Pid) -> Option<ActorInfo> {
     with_runtime(|inner| {
         let slot = inner.slot_at(pid)?;
@@ -141,11 +231,12 @@ pub fn actor_info(pid: Pid) -> Option<ActorInfo> {
     })
 }
 
-/// Build one `ActorInfo` for slot `idx`, or `None` if Vacant or
-/// racing-reclaimed. State is classified from a lock-free word load (the torn
-/// read); the cold lock then pins the generation (reclaim bumps it under that
-/// same lock) so the cold fields are coherent for this incarnation. `mail` is
-/// this slot's registry entry, if any.
+/// Build one `ActorInfo` for slot `idx`, or `None` if the slot is empty or
+/// was reclaimed while this read was in progress. The scheduling state comes
+/// from a lock-free word load (the source of the tearing described in the
+/// module docs); the per-actor lock then confirms the actor has not since
+/// exited and been replaced, so the rest of the fields are coherent for this
+/// exact actor. `mail` is this slot's registry entry, if any.
 fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorInfo> {
     let w = slot.state_word();
     let state = classify(w)?;
@@ -153,10 +244,11 @@ fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorI
     let pid = Pid::new(idx, gen);
 
     let cold = slot.cold.lock();
-    // If the generation moved between the lock-free load and acquiring the cold
-    // lock, the slot was reclaimed (and maybe reused) — drop it rather than mix
-    // one incarnation's state with another's cold data. (ps semantics: a racing
-    // actor may simply be missed mid-scan.)
+    // If the generation moved between the lock-free load and acquiring the
+    // per-actor lock, this actor exited (and the slot may already hold a new
+    // one). Drop it rather than mix one actor's state with another's data; a
+    // racing actor may simply be missed by this scan, which is expected (see
+    // the module docs' "Consistency" section).
     if word_gen(slot.state_word()) != gen {
         return None;
     }
@@ -172,7 +264,7 @@ fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorI
     let joiners = cold.waiters.len() as u32;
     drop(cold);
 
-    // Counters are hot-region atomics, read lock-free (RFC 016 Chunk 2).
+    // Counters are plain atomics, read lock-free.
     let overruns = slot.overruns();
     let messages_received = slot.messages_received();
     let budget_cycles = slot.budget_cycles();
@@ -202,39 +294,45 @@ fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorI
 }
 
 // ---------------------------------------------------------------------------
-// Chunk 3 — tree view (pure derivation over a Chunk-1 snapshot)
+// Tree view: a pure derivation over a snapshot
 // ---------------------------------------------------------------------------
 
-/// One node in the parentage forest. `children` are the actors whose recorded
-/// parent edge points at this node's pid.
+/// One node in the parentage forest returned by [`tree`]. `children` are the
+/// actors whose recorded parent (see [`ActorInfo::supervisor`]) points at
+/// this node's actor.
 #[derive(Debug, Clone)]
 pub struct TreeNode {
     pub info: ActorInfo,
-    /// The actor's recorded parent was absent from the snapshot (already
-    /// Done/Vacant, or itself a tombstone), so it was re-rooted under the forest
-    /// sentinel rather than dropped — the tree stays total (DECISION D8).
+    /// True if this actor's recorded parent was not found in the snapshot
+    /// (it had already exited, or was itself missing), so this node was
+    /// placed at the top of the forest instead of being dropped. This keeps
+    /// every actor in the snapshot visible somewhere in the tree, even one
+    /// whose parent is gone.
     pub orphaned: bool,
     pub children: Vec<TreeNode>,
 }
 
-/// The parentage forest. Roots are actors parented at `ROOT_PID` (genuine
-/// roots) plus re-rooted orphans. The edge is *spawned-by / parent*, not
-/// necessarily supervision (DECISION D9) — see [`ActorInfo::supervisor`].
+/// The parentage forest: every actor from a snapshot, arranged by who spawned
+/// whom. Roots are actors with no parent in the snapshot (including the
+/// run's own root actor) plus any orphaned actors (see [`TreeNode::orphaned`]).
+/// This mirrors spawn parentage, not necessarily a supervision tree; see
+/// [`ActorInfo::supervisor`].
 #[derive(Debug, Clone)]
 pub struct RuntimeTree {
     pub format_version: u16,
     pub roots: Vec<TreeNode>,
 }
 
-/// Take a live [`snapshot`] and fold it into the parentage forest.
+/// Take a fresh [`snapshot`] and fold it into the parentage forest.
 pub fn tree() -> RuntimeTree {
     tree_from(snapshot())
 }
 
-/// Fold an existing snapshot into a forest by grouping each actor under its
-/// parent pid — a single O(n) pass, no new reads. Exposed separately so a
-/// consumer that already holds a snapshot (or a synthetic one, in tests) can
-/// derive the tree without a second scan.
+/// Fold an existing snapshot into a parentage forest by grouping each actor
+/// under its parent, without taking a new snapshot. Useful if you already
+/// have one (for example, one built in a test, or one you took earlier and
+/// want to inspect again) and want the tree view of it without re-reading
+/// the runtime.
 pub fn tree_from(snap: RuntimeSnapshot) -> RuntimeTree {
     let RuntimeSnapshot { format_version, actors } = snap;
 
@@ -254,7 +352,7 @@ pub fn tree_from(snap: RuntimeSnapshot) -> RuntimeTree {
             children_of.entry(parent).or_default().push(i);
         } else {
             // Parent is the forest sentinel (genuine root) or absent from the
-            // snapshot (orphan, D8) — either way a root of the forest.
+            // snapshot (orphan): either way, a root of the forest.
             orphaned[i] = parent != ROOT_PID;
             roots.push(i);
         }

src/io.rs

@@ -13,44 +13,68 @@
 //! leaves the actor, no copying through an intermediary thread. Built on
 //! these are the conveniences `read(fd, &mut buf)` and `write(fd, &buf)`.
 //!
-//! Architecture
-//! ============
-//! Per `run()`, two OS threads:
-//!   - **epoll thread**: owns the epollfd. Loops in `epoll_wait`. On a
-//!     ready fd, pushes `Completion::FdReady { pid, fd, events }` to the
-//!     shared completion queue and writes the scheduler-wake pipe. On the
-//!     shutdown pipe (also registered in epollfd), exits.
-//!   - **pool thread**: blocks on the request mpsc. Runs the closure
-//!     inside `catch_unwind`, pushes `Completion::Blocking { pid, result }`,
-//!     writes the scheduler-wake pipe.
+//! Architecture (RFC 018: driver-enqueues)
+//! =======================================
+//! Per `run()`, two OS threads, each a *producer* behind the runtime's
+//! two-call contract — make the actor runnable (`unpark_at`, whose enqueue
+//! tail wakes a parked scheduler), nothing else:
 //!
-//! Both threads share a single `completions: Arc<Mutex<VecDeque<Completion>>>`
-//! and the same scheduler-wake pipe.
+//!   - **epoll thread**: owns `epoll_wait` on the epollfd. On a ready fd it
+//!     removes the parked waiter from the shared `waiters` map and DELs the
+//!     fd (both under the waiters lock — see below), then unparks the
+//!     actor directly. On the shutdown pipe (also registered in the
+//!     epollfd), exits.
+//!   - **pool thread**: blocks on the request mpsc. Runs the closure inside
+//!     `catch_unwind`, stashes the result in the actor's slot
+//!     (`pending_io_result`, under the cold lock, generation-checked),
+//!     decrements the runtime's `io_outstanding`, and unparks the actor.
 //!
-//! `epoll_ctl` (register/unregister fd interest) is called by the
-//! scheduler thread *directly* on the epollfd. That's well-defined per
-//! `epoll_ctl(2)`: a thread may be calling `epoll_wait` on the epollfd
-//! while another thread calls `epoll_ctl`. Avoids needing a second mpsc
-//! and a second wake mechanism.
+//! There is no shared completion queue and no wake pipe: each producer
+//! routes its own completion, so the whole byte-vs-completion visibility
+//! discipline of the drain era — and the stranded-completion hazards it
+//! defended against — is unrepresentable. Producers reach the runtime
+//! through a `Weak<RuntimeInner>`: upgraded per completion (the path is
+//! syscall-bound; the refcount op is noise) and avoiding an Arc cycle
+//! through `RuntimeInner::io`.
+//!
+//! `epoll_ctl` (register fd interest) is called by the scheduler thread
+//! directly on the epollfd. That's well-defined per `epoll_ctl(2)`: a
+//! thread may be calling `epoll_wait` on the epollfd while another thread
+//! calls `epoll_ctl`.
 //!
 //! Epoll mode
 //! ==========
 //! Level-triggered with EPOLLONESHOT. After a wakeup the kernel
 //! auto-disarms the fd, so we never get two wakeups for one
-//! `wait_readable` call. The scheduler explicitly `EPOLL_CTL_DEL`s the fd
-//! on completion to free the slot for re-registration. Net effect: each
+//! `wait_readable` call. The epoll thread explicitly `EPOLL_CTL_DEL`s the
+//! fd on readiness to free the slot for re-registration. Net effect: each
 //! `wait_readable(fd)` is one ADD, one wakeup, one DEL — symmetric and
 //! stateless between calls.
 //!
+//! ## The waiters lock is the ADD/DEL serialization
+//!
+//! Registration (scheduler thread: check-vacant, defensive DEL, ADD,
+//! insert) and readiness consumption (epoll thread: remove, DEL) each run
+//! entirely under the `waiters` mutex. This is what makes the
+//! oneshot-rearm race unrepresentable: a woken actor re-registering the
+//! same fd cannot interleave with the epoll thread's DEL for the *previous*
+//! registration — whichever takes the lock second sees a consistent
+//! kernel-side state. Lock order: `io` (the runtime's outer mutex, held by
+//! scheduler-side callers) → `waiters` → slot/queue leaves via `unpark_at`.
+//! The epoll thread takes `waiters` without `io` — it must never take
+//! `io`, both for lock-order hygiene and because teardown holds `io` while
+//! joining it.
+//!
 //! Fd hygiene
 //! ==========
 //! An actor stopped while waiting on an fd unwinds out of `wait_fd`'s park;
 //! a drop guard there (armed after a successful register, forgotten on a
-//! normal wake) removes the `waiters` entry iff it is still that wait's
-//! `(pid, epoch)` and only then `EPOLL_CTL_DEL`s the fd — an entry already
-//! consumed by a racing `FdReady` means the fd may carry someone else's
-//! fresh registration, which must be left alone. `epoll_register` keeps a
-//! defensive bare DEL before ADD as belt-and-braces.
+//! normal wake) calls [`IoThread::cancel_waiter`], which removes the
+//! `waiters` entry iff it is still that wait's `(pid, epoch)` and only then
+//! `EPOLL_CTL_DEL`s the fd — an entry already consumed by the epoll thread
+//! means the fd may carry someone else's fresh registration, which must be
+//! left alone. `epoll_register` keeps a defensive bare DEL before ADD as
+//! belt-and-braces.
 //!
 //! Buffers used with `read`/`write` should be on fds opened with
 //! `O_NONBLOCK`. If they aren't, the syscall may block the scheduler
@@ -68,13 +92,14 @@
 //! they have no equivalent panic-propagation path.
 
 use crate::pid::Pid;
+use crate::runtime::RuntimeInner;
 use std::any::Any;
-use std::collections::{HashMap, VecDeque};
+use std::collections::HashMap;
 use std::io;
 use std::os::fd::RawFd;
 use std::panic;
-use std::sync::mpsc;
-use std::sync::{Arc, Mutex};
+use std::sync::atomic::Ordering;
+use std::sync::{mpsc, Arc, Mutex, Weak};
 use std::thread::JoinHandle as OsJoinHandle;
 
 // ---------------------------------------------------------------------------
@@ -86,42 +111,29 @@ use std::thread::JoinHandle as OsJoinHandle;
 pub type IoResult = Result<Box<dyn Any + Send>, Box<dyn Any + Send>>;
 
 struct Request {
-    /// The submitter's park-epoch — carried through to the `Blocking`
-    /// completion so the wake is epoch-matched.
+    /// The submitter's park-epoch — the eventual wake is epoch-matched.
     epoch: u32,
     pid: Pid,
     /// The work to perform. Returns the wire-form result directly.
     work: Box<dyn FnOnce() -> IoResult + Send>,
 }
 
-/// Completion message from either IO thread back to the scheduler.
-pub enum Completion {
-    /// A `block_on_io` closure has finished (Ok = return value, Err = panic
-    /// payload).
-    Blocking { pid: Pid, epoch: u32, result: IoResult },
-    /// An fd registered via `wait_readable`/`wait_writable` is ready. The
-    /// scheduler looks up the parked pid in `waiters`, unparks it, and
-    /// removes the entry. `pid` isn't in this variant because the epoll
-    /// thread doesn't have access to the `waiters` map; the scheduler
-    /// thread owns that.
-    FdReady { fd: RawFd, events: u32 },
-}
+/// The parked-waiter map, shared between scheduler-side registration and
+/// the epoll thread's readiness consumption. See the module docs on why
+/// this single lock is the ADD/DEL serialization.
+type Waiters = Arc<Mutex<HashMap<RawFd, (Pid, u32)>>>;
 
 // ---------------------------------------------------------------------------
-// IoThread — created per `run()`, owned by `SchedulerState`.
+// IoThread — created per `run()`, owned by `RuntimeInner::io`.
 // ---------------------------------------------------------------------------
 
 pub struct IoThread {
-    // ----- Channels & queues -----
-
     /// Submission queue into the blocking-work pool.
     tx: mpsc::Sender<Request>,
-    /// Shared completion queue, fed by both the pool and the epoll thread.
-    completions: Arc<Mutex<VecDeque<Completion>>>,
-    /// Pipe the scheduler polls in its idle path. Both IO threads write to
-    /// `wake_write` after pushing a completion.
-    wake_read: RawFd,
-    wake_write: RawFd,
+    /// One parked actor per registered fd. Populated by `epoll_register`,
+    /// consumed by the epoll thread on readiness or `cancel_waiter` on an
+    /// unwound wait.
+    waiters: Waiters,
 
     // ----- Epoll machinery -----
 
@@ -133,39 +145,25 @@ pub struct IoThread {
     /// shutdown.
     shutdown_read: RawFd,
     shutdown_write: RawFd,
-    /// One parked actor per registered fd. Populated by `wait_readable` /
-    /// `wait_writable` and drained by the scheduler when a `FdReady`
-    /// completion is processed.
-    pub waiters: HashMap<RawFd, (Pid, u32)>,
 
     // ----- Threads -----
 
     pool_thread: Option<OsJoinHandle<()>>,
     epoll_thread: Option<OsJoinHandle<()>>,
-
-    /// Number of `block_on_io` requests in-flight. Used by the scheduler's
-    /// idle path to decide whether to wait on the pipe or exit. Fd waits
-    /// are not counted here; they're counted by `waiters.len()`.
-    pub outstanding: u32,
 }
 
 impl IoThread {
-    pub fn start() -> io::Result<Self> {
-        // Scheduler-facing wake pipe.
-        let (wake_read, wake_write) = make_pipe()?;
-        // Pool submission channel + shared completion queue.
+    /// Start the pool and epoll threads. `rt` is the producers' route back
+    /// into the runtime (slot table + unpark protocol); a `Weak` so the
+    /// `RuntimeInner → IoThread → RuntimeInner` cycle never forms.
+    pub(crate) fn start(rt: Weak<RuntimeInner>) -> io::Result<Self> {
+        // Pool submission channel.
         let (tx, rx) = mpsc::channel::<Request>();
-        let completions: Arc<Mutex<VecDeque<Completion>>> =
-            Arc::new(Mutex::new(VecDeque::new()));
+        let waiters: Waiters = Arc::new(Mutex::new(HashMap::new()));
 
         // Epoll machinery.
         let epollfd = unsafe { libc::epoll_create1(libc::EPOLL_CLOEXEC) };
         if epollfd < 0 {
-            // Best-effort fd cleanup before bailing.
-            unsafe {
-                libc::close(wake_read);
-                libc::close(wake_write);
-            }
             return Err(io::Error::last_os_error());
         }
 
@@ -174,8 +172,6 @@ impl IoThread {
             Err(e) => {
                 unsafe {
                     libc::close(epollfd);
-                    libc::close(wake_read);
-                    libc::close(wake_write);
                 }
                 return Err(e);
             }
@@ -202,79 +198,51 @@ impl IoThread {
                 libc::close(epollfd);
                 libc::close(shutdown_read);
                 libc::close(shutdown_write);
-                libc::close(wake_read);
-                libc::close(wake_write);
             }
             return Err(e);
         }
 
         // Spawn pool thread.
-        let pool_comps = completions.clone();
+        let pool_rt = rt.clone();
         let pool_thread = std::thread::Builder::new()
             .name("smarm-io-pool".into())
-            .spawn(move || pool_loop(rx, pool_comps, wake_write))?;
+            .spawn(move || pool_loop(rx, pool_rt))?;
 
         // Spawn epoll thread.
-        let epoll_comps = completions.clone();
+        let epoll_waiters = waiters.clone();
         let epoll_thread = std::thread::Builder::new()
             .name("smarm-io-epoll".into())
-            .spawn(move || epoll_loop(epollfd, epoll_comps, wake_write))?;
+            .spawn(move || epoll_loop(epollfd, epoll_waiters, rt))?;
 
         Ok(Self {
             tx,
-            completions,
-            wake_read,
-            wake_write,
+            waiters,
             epollfd,
             shutdown_read,
             shutdown_write,
-            waiters: HashMap::new(),
             pool_thread: Some(pool_thread),
             epoll_thread: Some(epoll_thread),
-            outstanding: 0,
         })
     }
 
-    /// Hand a request to the pool. Increments `outstanding`.
+    /// Hand a request to the pool. The caller (scheduler.rs) increments
+    /// `io_outstanding` BEFORE calling — the pool decrements on completion,
+    /// and an increment that trailed the completion would underflow.
     pub fn submit(&mut self, pid: Pid, epoch: u32, work: Box<dyn FnOnce() -> IoResult + Send>) {
-        self.outstanding += 1;
         // Send can only fail if the pool has hung up, which only happens
         // on shutdown. submit during shutdown is a bug.
-        self.tx
-            .send(Request { pid, epoch, work })
-            .expect("io pool hung up unexpectedly");
-    }
-
-    /// Drain every available completion. Caller (the scheduler) routes the
-    /// results and updates `outstanding` / `waiters` accordingly.
-    pub fn drain_completions(&mut self) -> Vec<Completion> {
-        let mut q = self.completions.lock().unwrap();
-        let mut out = Vec::with_capacity(q.len());
-        while let Some(c) = q.pop_front() {
-            out.push(c);
+        if self.tx.send(Request { pid, epoch, work }).is_err() {
+            panic!("smarm: io pool hung up unexpectedly (submit during shutdown)");
         }
-        out
-    }
-
-    pub fn wake_fd(&self) -> RawFd {
-        self.wake_read
-    }
-
-    /// Write the wake pipe directly: rouse every scheduler thread blocked in
-    /// its idle `poll_wake`. Used by the terminal (AllDone) path — an idle
-    /// sibling may be blocked on a snapshot that nothing will ever refresh
-    /// (an orphaned timer deadline, or `io_outstanding` from a waiter that
-    /// was stop-cancelled and so never produces a completion).
-    pub fn wake(&self) {
-        wake_scheduler(self.wake_write);
     }
 
     /// Register interest in `fd` becoming readable/writable; record `pid`
-    /// as the parked waiter. The epoll thread will push a `FdReady`
-    /// completion when the kernel signals.
+    /// as the parked waiter. The epoll thread unparks it on readiness.
+    /// The caller increments `io_fd_waiters` BEFORE calling (mirror of
+    /// `submit`'s contract) and decrements it again if this errors.
     ///
-    /// EPOLLONESHOT: one wakeup per registration. The scheduler must
-    /// `epoll_del` on completion to free the slot for re-registration.
+    /// EPOLLONESHOT: one wakeup per registration; the epoll thread DELs on
+    /// readiness, `cancel_waiter` DELs on an unwound wait.
     pub fn epoll_register(
         &mut self,
         fd: RawFd,
@@ -283,20 +251,24 @@ impl IoThread {
         readable: bool,
         writable: bool,
     ) -> io::Result<()> {
+        let mut waiters = match self.waiters.lock() {
+            Ok(g) => g,
+            Err(e) => panic!("smarm: io waiters lock poisoned (core corrupt): {e}"),
+        };
         // Two actors waiting on the same fd would be a misuse: the kernel
         // delivers exactly one EPOLLONESHOT wakeup, so the second waiter
         // would hang. Reject up front.
-        if self.waiters.contains_key(&fd) {
+        if waiters.contains_key(&fd) {
             return Err(io::Error::new(
                 io::ErrorKind::AlreadyExists,
                 "fd already has a parked waiter",
             ));
         }
 
-        // Belt-and-braces: the unwind guard in `wait_fd` is responsible for
-        // cleaning up a stopped waiter's registration, but a bare DEL is
-        // harmless if the fd isn't registered (ENOENT) and removes any leak
-        // a path we haven't thought of might leave behind.
+        // Belt-and-braces: `cancel_waiter` is responsible for cleaning up a
+        // stopped waiter's registration, but a bare DEL is harmless if the
+        // fd isn't registered (ENOENT) and removes any leak a path we
+        // haven't thought of might leave behind.
         unsafe {
             libc::epoll_ctl(self.epollfd, libc::EPOLL_CTL_DEL, fd, std::ptr::null_mut());
         }
@@ -318,19 +290,29 @@ impl IoThread {
         if r < 0 {
             return Err(io::Error::last_os_error());
         }
-        self.waiters.insert(fd, (pid, epoch));
+        waiters.insert(fd, (pid, epoch));
         Ok(())
     }
 
-    /// Remove `fd` from the epollfd. Called by the scheduler after a
-    /// `FdReady` completion, so the next `wait_readable(fd)` can ADD again.
-    ///
-    /// Does NOT touch `waiters` — that's the scheduler's bookkeeping; this
-    /// is purely the kernel-side cleanup.
-    pub fn epoll_deregister(&mut self, fd: RawFd) {
-        // EPOLL_CTL_DEL of an already-removed fd returns ENOENT; ignore.
-        unsafe {
-            libc::epoll_ctl(self.epollfd, libc::EPOLL_CTL_DEL, fd, std::ptr::null_mut());
+    /// Remove `fd`'s waiter iff it is still `(pid, epoch)`, DELing the fd
+    /// from the epollfd in the same critical section. Returns whether the
+    /// entry was removed (the caller then decrements `io_fd_waiters`).
+    /// `false` means the epoll thread consumed the registration first —
+    /// the fd may already carry someone else's fresh ADD; hands off.
+    pub fn cancel_waiter(&mut self, fd: RawFd, pid: Pid, epoch: u32) -> bool {
+        let mut waiters = match self.waiters.lock() {
+            Ok(g) => g,
+            Err(e) => panic!("smarm: io waiters lock poisoned (core corrupt): {e}"),
+        };
+        if waiters.get(&fd) == Some(&(pid, epoch)) {
+            waiters.remove(&fd);
+            // EPOLL_CTL_DEL of an already-removed fd returns ENOENT; ignore.
+            unsafe {
+                libc::epoll_ctl(self.epollfd, libc::EPOLL_CTL_DEL, fd, std::ptr::null_mut());
+            }
+            true
+        } else {
+            false
         }
     }
 }
@@ -351,7 +333,10 @@ impl Drop for IoThread {
         let real_tx = std::mem::replace(&mut self.tx, dead_tx);
         drop(real_tx);
 
-        // 3. Join both threads.
+        // 3. Join both threads. Safe even while the caller holds the
+        //    runtime's `io` mutex: neither thread ever takes it (they reach
+        //    the runtime through a Weak they upgrade per completion, and
+        //    the epoll thread's only lock is `waiters`).
         if let Some(h) = self.epoll_thread.take() {
             let _ = h.join();
         }
@@ -364,8 +349,6 @@ impl Drop for IoThread {
             libc::close(self.epollfd);
             libc::close(self.shutdown_read);
             libc::close(self.shutdown_write);
-            libc::close(self.wake_read);
-            libc::close(self.wake_write);
         }
     }
 }
@@ -376,36 +359,38 @@ impl Drop for IoThread {
 const SHUTDOWN_EPOLL_TOKEN: u64 = u64::MAX;
 
 // ---------------------------------------------------------------------------
-// Pool loop
+// Pool loop (producer: Blocking completions)
 // ---------------------------------------------------------------------------
 
-fn pool_loop(
-    rx: mpsc::Receiver<Request>,
-    completions: Arc<Mutex<VecDeque<Completion>>>,
-    wake_write: RawFd,
-) {
+fn pool_loop(rx: mpsc::Receiver<Request>, rt: Weak<RuntimeInner>) {
     while let Ok(Request { pid, epoch, work }) = rx.recv() {
         let result: IoResult = match panic::catch_unwind(panic::AssertUnwindSafe(work)) {
             Ok(r) => r,
             Err(payload) => Err(payload),
         };
-        completions
-            .lock()
-            .unwrap()
-            .push_back(Completion::Blocking { pid, epoch, result });
-        wake_scheduler(wake_write);
+        let Some(inner) = rt.upgrade() else { return };
+        // Stash the result under the cold lock (generation-checked: an
+        // actor stopped with the op in flight discards it), decrement the
+        // in-flight count, then wake through the epoch-matched unpark. The
+        // unpark's enqueue tail wakes a parked scheduler; the actor stays
+        // `live` until it resumes and finalizes, so the decrement's
+        // ordering against the termination verdict is not load-bearing.
+        if let Some(slot) = inner.slot_at(pid) {
+            let mut cold = slot.cold.lock();
+            if slot.generation() == pid.generation() {
+                cold.pending_io_result = Some(result);
+            }
+        }
+        inner.io_outstanding.fetch_sub(1, Ordering::AcqRel);
+        inner.unpark_at(pid, epoch);
     }
 }
 
 // ---------------------------------------------------------------------------
-// Epoll loop
+// Epoll loop (producer: FdReady completions)
 // ---------------------------------------------------------------------------
 
-fn epoll_loop(
-    epollfd: RawFd,
-    completions: Arc<Mutex<VecDeque<Completion>>>,
-    wake_write: RawFd,
-) {
+fn epoll_loop(epollfd: RawFd, waiters: Waiters, rt: Weak<RuntimeInner>) {
     // Buffer for epoll_wait. 64 is plenty for our scale; if a real load
     // appears that needs more, this is a one-line change.
     const MAX_EVENTS: usize = 64;
@@ -433,26 +418,41 @@ fn epoll_loop(
         }
 
         let mut shutdown_requested = false;
-        let mut pushed_any = false;
-        {
-            let mut q = completions.lock().unwrap();
-            for ev in events.iter().take(n as usize) {
-                if ev.u64 == SHUTDOWN_EPOLL_TOKEN {
-                    shutdown_requested = true;
-                    continue;
+        for ev in events.iter().take(n as usize) {
+            if ev.u64 == SHUTDOWN_EPOLL_TOKEN {
+                shutdown_requested = true;
+                continue;
+            }
+            let fd = ev.u64 as RawFd;
+            // Consume the registration: remove + DEL under the waiters
+            // lock (the ADD/DEL serialization — see module docs). A
+            // vanished entry means `cancel_waiter` beat us: the wake is
+            // already moot.
+            let entry = {
+                let mut w = match waiters.lock() {
+                    Ok(g) => g,
+                    Err(e) => {
+                        panic!("smarm: io waiters lock poisoned (core corrupt): {e}")
+                    }
+                };
+                let entry = w.remove(&fd);
+                if entry.is_some() {
+                    unsafe {
+                        libc::epoll_ctl(
+                            epollfd,
+                            libc::EPOLL_CTL_DEL,
+                            fd,
+                            std::ptr::null_mut(),
+                        );
+                    }
                 }
-                let fd = ev.u64 as RawFd;
-                let evs = ev.events;
-                q.push_back(Completion::FdReady {
-                    fd,
-                    events: evs,
-                });
-                pushed_any = true;
+                entry
+            };
+            if let Some((pid, epoch)) = entry {
+                let Some(inner) = rt.upgrade() else { return };
+                inner.io_fd_waiters.fetch_sub(1, Ordering::AcqRel);
+                inner.unpark_at(pid, epoch);
             }
-        }
-
-        if pushed_any {
-            wake_scheduler(wake_write);
         }
         if shutdown_requested {
             return;
@@ -460,27 +460,8 @@ fn epoll_loop(
     }
 }
 
-/// Write one byte to the scheduler's wake pipe. Retries on EINTR; ignores
-/// EAGAIN (pipe full means there's already an outstanding wake we haven't
-/// consumed yet, which is sufficient).
-fn wake_scheduler(wake_write: RawFd) {
-    let buf: [u8; 1] = [0];
-    unsafe {
-        loop {
-            let n = libc::write(wake_write, buf.as_ptr() as *const _, 1);
-            if n < 0 {
-                let e = *libc::__errno_location();
-                if e == libc::EINTR {
-                    continue;
-                }
-            }
-            break;
-        }
-    }
-}
-
 // ---------------------------------------------------------------------------
-// Pipe helpers (unchanged from v0.2)
+// Pipe helper
 // ---------------------------------------------------------------------------
 
 fn make_pipe() -> io::Result<(RawFd, RawFd)> {
@@ -491,46 +472,3 @@ fn make_pipe() -> io::Result<(RawFd, RawFd)> {
     }
     Ok((fds[0], fds[1]))
 }
-
-/// Drain pending bytes from the wake pipe. The scheduler calls this after
-/// a `poll` wakeup so the next idle call sees an empty pipe.
-pub fn drain_wake_pipe(fd: RawFd) {
-    let mut buf = [0u8; 64];
-    loop {
-        let n = unsafe { libc::read(fd, buf.as_mut_ptr() as *mut _, buf.len()) };
-        if n <= 0 {
-            break;
-        }
-    }
-}
-
-/// Block on `fd` for up to `timeout`, returning when either there's data
-/// to read or the timeout elapses. `None` for `timeout` means wait forever.
-pub fn poll_wake(fd: RawFd, timeout: Option<std::time::Duration>) {
-    let timeout_ms: libc::c_int = match timeout {
-        None => -1,
-        Some(d) => {
-            let ms = d.as_millis();
-            if ms > i32::MAX as u128 {
-                i32::MAX
-            } else {
-                ms as i32
-            }
-        }
-    };
-    let mut pfd = libc::pollfd {
-        fd,
-        events: libc::POLLIN,
-        revents: 0,
-    };
-    loop {
-        let r = unsafe { libc::poll(&mut pfd as *mut _, 1, timeout_ms) };
-        if r < 0 {
-            let e = unsafe { *libc::__errno_location() };
-            if e == libc::EINTR {
-                continue;
-            }
-        }
-        break;
-    }
-}

src/lib.rs

@@ -32,12 +32,14 @@ pub mod introspect;
 #[cfg(feature = "observer")]
 pub mod observer;
 pub mod runtime;
+pub(crate) mod park;
 pub(crate) mod raw_mutex;
 pub(crate) mod slot_state;
 pub(crate) mod sync_shim;
 #[doc(hidden)] // pub only so benches/rq_micro.rs can drive the raw structures
 pub mod run_queue;
 pub mod trace;
+pub mod causal;
 
 // ---------------------------------------------------------------------------
 // Global allocator
@@ -79,7 +81,8 @@ pub use registry::{
 };
 pub use runtime::{init, Config, Runtime};
 pub use scheduler::{
-    block_on_io, cancel_timer, request_stop, run, self_pid, send_after, send_after_named, sleep,
+    block_on_io, cancel_timer, request_stop, run, self_pid, send_after, send_after_named,
+    send_after_named_wall, send_after_wall, sleep, sleep_wall,
     spawn, spawn_addr, spawn_under, wait_readable, wait_readable_timeout, wait_writable,
     wait_writable_timeout, yield_now, FdArm, JoinError, JoinHandle,
 };

src/link.rs

@@ -135,7 +135,10 @@ pub fn link<A>(target: Pid<A>) {
 
     if registered_on_target {
         with_runtime(|inner| {
-            let slot = inner.slot_at(me).expect("link: own slot vanished");
+            let slot = match inner.slot_at(me) {
+                Some(s) => s,
+                None => panic!("smarm: link own slot vanished (core corrupt)"),
+            };
             let mut cold = slot.cold.lock();
             if !cold.links.contains(&target) {
                 cold.links.push(target);

src/monitor.rs

@@ -1,49 +1,85 @@
-//! Process monitors.
+//! Find out when another actor dies, without it knowing or caring that you're
+//! watching.
 //!
-//! `monitor(target)` asks the runtime to deliver a single [`Down`] when
-//! `target` terminates, and hands back a [`Monitor`] — the [`Receiver`] to read
-//! it from, plus the identity (`id`, `target`) needed to take the registration
-//! back down with [`demonitor`]. A monitor is:
+//! Say one actor manages a pool of workers and needs to know when a worker
+//! exits, so it can replace it. The worker does not need to know it is being
+//! watched, and nothing about the worker's own behavior should change because
+//! someone is watching it. That is what [`monitor`] is for: call
+//! `monitor(target)` to get a [`Monitor`], and read exactly one [`Down`]
+//! message off `monitor.rx` whenever `target` terminates, however it
+//! terminates.
 //!
-//!   - **unidirectional** — the watcher learns of the target's death, but the
-//!     target learns nothing of the watcher, and the watcher is unaffected by
-//!     the death beyond the notification (contrast a *link*, which propagates
-//!     failure);
-//!   - **one-shot** — exactly one `Down` is ever sent for a given monitor.
-//!     The returned channel closes afterwards, so a second `recv()` yields
-//!     `Err(RecvError)`.
+//! ```
+//! use smarm::{monitor, run, spawn, DownReason};
 //!
-//! This generalizes the older single-`supervisor_channel` mechanism: a
-//! supervisor is just a hard-wired monitor that the parent installs at spawn
-//! time. Here any actor may monitor any pid, any number of times.
+//! run(|| {
+//!     let worker = spawn(|| {
+//!         // does some work, then returns
+//!     });
+//!     let pid = worker.pid();
 //!
-//! ## Reasons
+//!     let m = monitor(pid);
+//!     let _ = worker.join();
 //!
-//! [`DownReason`] is deliberately payload-free. A panicking actor's payload
-//! has a single owner and is delivered to whoever `join()`s the actor (as
-//! `JoinError`); a monitor only learns *that* it panicked, not the value.
-//! Monitoring a pid that is already gone (reclaimed, or never alive) yields
-//! [`DownReason::NoProc`] immediately, mirroring Erlang's `noproc`.
+//!     let down = m.rx.recv().expect("monitor channel closed before Down");
+//!     assert_eq!(down.pid, pid);
+//!     assert_eq!(down.reason, DownReason::Exit);
+//! });
+//! ```
 //!
-//! ## Demonitoring
+//! A monitor is one-directional and one-shot:
 //!
-//! Each `monitor()` registration is tagged with a process-unique [`MonitorId`].
-//! [`demonitor`] removes the registration named by a [`Monitor`] from its
-//! target's slot, returning `Some(id)` if a live registration was found or
-//! `None` if it had already fired (or the target is gone). Dropping the
-//! [`Monitor`] afterwards discards any `Down` that the target had *already*
-//! queued — the equivalent of Erlang's `demonitor(Ref, [flush])`.
+//! - **One-directional**: the watcher learns that the target died, but the
+//!   target is completely unaffected. It never learns it was being watched,
+//!   and its own behavior and lifetime do not change because of the monitor.
+//!   This is the opposite of a [`link`](mod@crate::link), which is bidirectional:
+//!   linking two actors means an abnormal death on either side can bring the
+//!   other down too. Reach for a monitor when you just want to *know*; reach
+//!   for a link when a peer's crash should actually stop you.
+//! - **One-shot**: you get exactly one [`Down`] per `monitor()` call, then the
+//!   channel closes. Calling `monitor` again on the same target (or a
+//!   different one) gives you an independent registration with its own
+//!   [`Monitor`] and its own one-shot channel; nothing stops you from
+//!   monitoring the same actor many times over; each call is watched and
+//!   fires on its own.
 //!
-//! ## Races
+//! ## Why a monitor never hands you the panic value
 //!
-//! Registration (below) and `finalize_actor` (in `runtime`) both run under the
-//! shared-state mutex, so a target that is still alive when its monitor is
-//! registered is guaranteed to deliver a real `Down`; there is no window in
-//! which the death slips between the liveness check and the registration.
-//! `demonitor` is protected by the generation half of the pid: if the target
-//! has died and its slot index been recycled, `slot_mut(target)` fails the
-//! generation check and `demonitor` is a clean no-op — it can never strip a
-//! *different* actor's monitor that happens to share the slot index.
+//! If the target panicked, [`Down`] tells you *that* it panicked
+//! ([`DownReason::Panic`]), but not the panic's payload. The payload has a
+//! single owner: it is handed to whichever caller `join()`s the actor's
+//! [`JoinHandle`](crate::JoinHandle), as a `JoinError`. A monitor only needs
+//! to know that something went wrong, not reproduce the exact value that
+//! caused it, so it gets the reason and nothing else.
+//!
+//! Monitoring a target that is already gone (it finished and was cleaned up,
+//! or the pid never pointed at a real actor) is not an error: you get a
+//! [`Down`] with [`DownReason::NoProc`] right away, instead of waiting
+//! forever for something that already happened.
+//!
+//! ## Stopping a monitor early
+//!
+//! [`demonitor`] cancels a monitor before it fires. If the registration was
+//! still live, it removes it and returns `Some` of the monitor's id: no
+//! `Down` will arrive on that channel from here on. If the target had already
+//! died and its `Down` already sent, there is nothing left to cancel and
+//! `demonitor` returns `None`; the `Down` you already have (or that is
+//! already sitting in the channel) is unaffected.
+//!
+//! If you want to cancel *and* make sure a `Down` that already arrived is
+//! discarded without reading it, just drop the [`Monitor`]: dropping it closes
+//! its receiver, and any queued `Down` is dropped along with it.
+//!
+//! ## Correctness notes for implementers
+//!
+//! A target that is still alive at the moment `monitor()` registers is
+//! guaranteed to eventually produce a real `Down`: registration and the
+//! target's own termination bookkeeping run under the same lock, so there is
+//! no window in which the target could die without the just-added
+//! registration seeing it. `demonitor` is similarly race-free against a target
+//! that has since died and had its slot reused by a new, unrelated actor: it
+//! is checked against the exact monitored incarnation, so it can never remove
+//! a different actor's registration by accident, it simply reports `None`.
 
 use crate::channel::{channel, Receiver, Sender};
 use crate::pid::Pid;
@@ -51,8 +87,8 @@ use crate::scheduler::with_runtime;
 
 /// Why a monitored actor went down.
 ///
-/// `Copy` because it carries no payload — see the module docs for why the
-/// panic payload is *not* included here.
+/// Carries no payload: see the module docs for why a monitor never receives
+/// the panic value itself.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum DownReason {
     /// The target returned normally.
@@ -76,21 +112,22 @@ pub struct Down {
     pub reason: DownReason,
 }
 
-/// A process-unique identifier for one `monitor()` registration.
+/// A unique identifier for one [`monitor`] registration.
 ///
-/// Opaque and `Copy`. Allocated from a monotonic counter in shared state, so
-/// it is never reused for the lifetime of the runtime — distinct `monitor()`
-/// calls on the same target get distinct ids, which is what lets [`demonitor`]
-/// tear down exactly one of several monitors on a target.
+/// Opaque and `Copy`. Never reused for the life of the runtime, so if you
+/// monitor the same target more than once, each call's id is distinct. This
+/// is what lets [`demonitor`] tear down exactly one of several monitors on
+/// the same target without disturbing the others.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct MonitorId(pub(crate) u64);
 
 /// A live monitor: the receiving end of the one-shot [`Down`] channel, plus the
 /// identity needed to [`demonitor`] it.
 ///
-/// Read the notification from [`Monitor::rx`]. Not `Clone` (the receiver is a
-/// single consumer). Dropping it closes the receiving end; if a `Down` was
-/// already queued it is discarded with the channel.
+/// Read the notification from [`Monitor::rx`]. Not `Clone`, since only one
+/// side is meant to consume it. Dropping a `Monitor` closes the receiving
+/// end; if a `Down` had already arrived but was never read, it is discarded
+/// along with it.
 pub struct Monitor {
     /// This registration's process-unique id.
     pub id: MonitorId,
@@ -110,10 +147,11 @@ pub fn monitor<A>(target: Pid<A>) -> Monitor {
     let target = target.erase();
     let (tx, rx) = channel::<Down>();
 
-    // Register under the target's cold lock. `tx.clone()` takes the channel's
-    // own lock — a Channel-class RawMutex, explicitly permitted *under* a Leaf
-    // (cold) lock by the lock order (see raw_mutex.rs). We must still not
-    // *send* under the lock, as `Sender::send` can unpark a parked receiver,
+    // Implementation note: registration happens under the target's cold
+    // lock. `tx.clone()` takes the channel's own lock, a Channel-class
+    // RawMutex, which is explicitly permitted under a Leaf (cold) lock by
+    // the lock order documented in raw_mutex.rs. We must still not *send*
+    // under the lock, since `Sender::send` can unpark a parked receiver,
     // and there's no reason to nest that.
     let (id, registered) = with_runtime(|inner| {
         let id = inner.alloc_monitor_id();
@@ -140,19 +178,21 @@ pub fn monitor<A>(target: Pid<A>) -> Monitor {
 }
 
 /// Cancel the monitor `m`. Returns `Some(id)` if a live registration was found
-/// on the target's slot and removed, or `None` if there was nothing to remove
-/// — the target already fired its `Down` (the registration is drained on
-/// finalize), was never alive (`NoProc`), or has been reclaimed.
+/// and removed, so no `Down` will arrive on `m.rx` from here on. Returns
+/// `None` if there was nothing left to remove: the target had already gone
+/// down and its `Down` was already sent (or is already sitting in the
+/// channel, unread).
 ///
-/// This stops any *future* `Down`. To also discard a `Down` the target may have
-/// *already* queued (the finalize-races-demonitor case), drop `m` afterwards;
-/// dropping the [`Monitor`] closes its receiver and the queued notice goes with
-/// it — the analogue of Erlang's `demonitor(Ref, [flush])`.
+/// This only stops a *future* `Down`. If you also want to discard a `Down`
+/// that already arrived (or is about to, in a race with this call), drop `m`
+/// instead of, or in addition to, calling this: dropping the [`Monitor`]
+/// closes its receiver and any queued notice is discarded with it.
 pub fn demonitor(m: &Monitor) -> Option<MonitorId> {
-    // Remove the registration under the target's cold lock, but move the
-    // `Sender` *out* and let it drop only after the lock is released:
-    // dropping the last sender runs `Sender::drop`, which may unpark a parked
-    // receiver — legal under a cold lock, but pointless to nest.
+    // Implementation note: the registration is removed under the target's
+    // cold lock, but the `Sender` is moved *out* and dropped only after the
+    // lock is released. Dropping the last sender runs `Sender::drop`, which
+    // may unpark a parked receiver; legal under a cold lock, but pointless
+    // to nest.
     let removed: Option<(MonitorId, Sender<Down>)> = with_runtime(|inner| {
         let slot = inner.slot_at(m.target)?;
         let mut cold = slot.cold.lock();

src/mutex.rs

@@ -1,12 +1,89 @@
-//! Actor-aware mutex with mandatory timeout.
+//! Shared mutable state across actors, when a channel is overkill.
 //!
-//! `Mutex<T>` parks the calling *green* thread on contention rather than
-//! blocking the OS thread. Every lock attempt is bounded by a timeout.
+//! smarm actors normally coordinate by sending messages, and for a piece of
+//! owned state the right tool is usually a `gen_server`: one actor holds the
+//! data and everyone else talks to it. Sometimes that is more machinery than
+//! you need, and plain shared, lockable state is simpler: [`Mutex<T>`] is
+//! that escape hatch. It behaves like `std::sync::Mutex<T>`, guarding a value
+//! of type `T` behind a guard that gives you `&mut T` while held, but it is
+//! built for smarm's actors rather than OS threads.
 //!
-//! Internals use `Arc<std::sync::Mutex<...>>` so the type is genuinely
-//! `Send + Sync` and can be shared across scheduler threads.
+//! The key difference from `std::sync::Mutex` is what happens on contention.
+//! [`Mutex::lock`] parks the calling actor (a cooperatively scheduled green
+//! thread) rather than blocking the underlying OS thread, so other actors on
+//! the same OS thread keep running while it waits. And every lock attempt is
+//! bounded by a timeout: an actor that hangs on to the lock forever (stuck in
+//! a bug, or just slow) would otherwise wedge every other actor waiting on
+//! it, so smarm makes the wait bounded by default instead of leaving it up
+//! to you to remember.
 //!
-//! Fairness: FIFO. Poisoning: none. Reentrance: deadlock (caller bug).
+//! ## A first lock
+//!
+//! ```
+//! use smarm::{run, spawn, Mutex};
+//!
+//! run(|| {
+//!     let counter = Mutex::new(0u32);
+//!
+//!     // Mutex::clone() is cheap and hands out another handle to the SAME
+//!     // underlying value, much like Arc::clone: every clone shares one lock
+//!     // and one value, so mutations through one are visible through all.
+//!     let a = counter.clone();
+//!     let b = counter.clone();
+//!
+//!     let h1 = spawn(move || {
+//!         let mut guard = a.lock().unwrap();
+//!         *guard += 1;
+//!     });
+//!     let h2 = spawn(move || {
+//!         let mut guard = b.lock().unwrap();
+//!         *guard += 1;
+//!     });
+//!     h1.join().unwrap();
+//!     h2.join().unwrap();
+//!
+//!     assert_eq!(*counter.lock().unwrap(), 2);
+//! });
+//! ```
+//!
+//! ## Choosing a timeout
+//!
+//! [`Mutex::lock`] waits up to [`DEFAULT_TIMEOUT`] (30 seconds) before giving
+//! up with [`LockTimeout`]. To use a different bound for one call, use
+//! [`Mutex::lock_timeout`] instead; to change the default for every future
+//! `lock()` call on this mutex (including through its clones), use
+//! [`Mutex::set_default_timeout`]. If you never want to wait at all, use
+//! [`Mutex::try_lock`], which returns immediately whether or not the lock was
+//! free.
+//!
+//! ## Fairness and panics
+//!
+//! Waiters are granted the lock in the order they started waiting (FIFO), so
+//! no actor can be starved by later arrivals repeatedly cutting in line.
+//!
+//! This mutex never poisons. `std::sync::Mutex` marks itself poisoned if a
+//! thread panics while holding the lock, because a partly mutated value might
+//! be left behind for the next lock holder to see. smarm's actors already
+//! rely on `Drop` running during unwinding to release the lock, so if a
+//! holder panics, [`MutexGuard::drop`] still runs and the next waiter is
+//! granted the lock normally. It is the same tradeoff `std::sync::Mutex`
+//! offers you if you choose to ignore poisoning: you may see a value left
+//! mid-update by the panicking actor, so a panic inside a critical section is
+//! still a bug worth fixing, just not one that also wedges every future lock
+//! attempt.
+//!
+//! Locking a mutex you already hold (on the same actor) does not queue
+//! behind yourself: it deadlocks, the same way relocking a non-reentrant
+//! `std::sync::Mutex` does. Don't call `lock` while already holding a guard
+//! from the same `Mutex`.
+//!
+//! ## Outside the runtime
+//!
+//! `Mutex<T>` also works when called from plain code that is not running as
+//! a smarm actor (for example, in a test's setup code before calling
+//! [`run`](crate::run)). There, an actor's cooperative park has no meaning,
+//! so a lock attempt instead blocks the calling OS thread directly until the
+//! mutex is free; there is no timeout on this path.
 
 use crate::pid::Pid;
 use crate::scheduler;
@@ -15,8 +92,14 @@ use std::collections::VecDeque;
 use std::sync::{Arc, Mutex as StdMutex};
 use std::time::Duration;
 
+/// How long [`Mutex::lock`] waits for the lock before giving up, unless
+/// overridden per-mutex with [`Mutex::set_default_timeout`] or per-call with
+/// [`Mutex::lock_timeout`].
 pub const DEFAULT_TIMEOUT: Duration = Duration::from_secs(30);
 
+/// Returned by [`Mutex::lock`] / [`Mutex::lock_timeout`] when the timeout
+/// elapses before the lock became available. The lock attempt is abandoned;
+/// nothing was acquired, and the mutex's value is unaffected.
 #[derive(Debug, PartialEq, Eq, Clone, Copy)]
 pub struct LockTimeout;
 
@@ -64,20 +147,23 @@ impl MutexCore {
 impl TimerTarget for MutexCore {
     fn on_timeout(&self, pid: Pid, epoch: u32) {
         let unpark = {
-            let mut st = self.state.lock().unwrap();
+            let mut st = match self.state.lock() {
+                Ok(g) => g,
+                Err(e) => panic!("smarm: mutex state lock poisoned (core corrupt): {e}"),
+            };
             // Remove from waiters only if still there with matching epoch.
             // If the lock was already granted (holder == Some(pid)), the
-            // timer fired after the grant — treat as no-op; the actor
+            // timer fired after the grant: treat as no-op; the actor
             // will see `is_holder == true` and return Ok.
             if st.holder == Some(pid) {
                 return;
             }
-            let pos = st.waiters.iter().position(|w| w.pid == pid && w.epoch == epoch);
-            if pos.is_some() {
-                st.waiters.remove(pos.unwrap());
-                true
-            } else {
-                false
+            match st.waiters.iter().position(|w| w.pid == pid && w.epoch == epoch) {
+                Some(pos) => {
+                    st.waiters.remove(pos);
+                    true
+                }
+                None => false,
             }
         };
         if unpark {
@@ -97,6 +183,8 @@ pub struct Mutex<T> {
 }
 
 impl<T> Mutex<T> {
+    /// Wrap `value` in a new mutex, initially unlocked, with the default
+    /// lock timeout ([`DEFAULT_TIMEOUT`]).
     pub fn new(value: T) -> Self {
         Self {
             core: Arc::new(MutexCore::new(DEFAULT_TIMEOUT)),
@@ -104,15 +192,36 @@ impl<T> Mutex<T> {
         }
     }
 
+    /// Change how long future [`lock`](Self::lock) calls on this mutex wait
+    /// before giving up. Applies to every clone of this `Mutex` (they share
+    /// one underlying lock), and to `lock` calls already in progress that
+    /// have not yet started waiting. Does not affect [`lock_timeout`](Self::lock_timeout)
+    /// calls, which always use the timeout passed in.
     pub fn set_default_timeout(&self, timeout: Duration) {
-        self.core.state.lock().unwrap().default_timeout = timeout;
+        match self.core.state.lock() {
+            Ok(mut st) => st.default_timeout = timeout,
+            Err(e) => panic!("smarm: mutex state lock poisoned (core corrupt): {e}"),
+        }
     }
 
+    /// Acquire the lock, waiting up to this mutex's default timeout
+    /// ([`DEFAULT_TIMEOUT`], or whatever [`set_default_timeout`](Self::set_default_timeout)
+    /// last set) if it is currently held elsewhere. Returns a [`MutexGuard`]
+    /// that releases the lock when dropped, or [`LockTimeout`] if the
+    /// deadline passes first. To use a one-off timeout instead of the
+    /// mutex's default, call [`lock_timeout`](Self::lock_timeout) directly.
     pub fn lock(&self) -> Result<MutexGuard<'_, T>, LockTimeout> {
-        let timeout = self.core.state.lock().unwrap().default_timeout;
+        let timeout = match self.core.state.lock() {
+            Ok(st) => st.default_timeout,
+            Err(e) => panic!("smarm: mutex state lock poisoned (core corrupt): {e}"),
+        };
         self.lock_timeout(timeout)
     }
 
+    /// Acquire the lock, waiting up to `timeout` (ignoring this mutex's
+    /// default) if it is currently held elsewhere. Returns a [`MutexGuard`]
+    /// that releases the lock when dropped, or [`LockTimeout`] if `timeout`
+    /// elapses first with the lock still unavailable.
     pub fn lock_timeout(&self, timeout: Duration) -> Result<MutexGuard<'_, T>, LockTimeout> {
         // Outside the runtime (e.g. in tests, after run() returns) there is no
         // current actor PID.  Fall back to a blocking std::sync::Mutex acquire.
@@ -122,12 +231,21 @@ impl<T> Mutex<T> {
 
         // Fast path: nobody holds it.
         {
-            let mut st = self.core.state.lock().unwrap();
+            let mut st = match self.core.state.lock() {
+                Ok(g) => g,
+                Err(e) => panic!("smarm: mutex state lock poisoned (core corrupt): {e}"),
+            };
             if st.holder.is_none() {
                 st.holder = Some(me);
                 drop(st);
-                let value = self.value.lock().unwrap().take()
-                    .expect("Mutex: value missing on free fast path");
+                let taken = match self.value.lock() {
+                    Ok(mut g) => g.take(),
+                    Err(e) => panic!("smarm: mutex value lock poisoned (core corrupt): {e}"),
+                };
+                let value = match taken {
+                    Some(v) => v,
+                    None => panic!("smarm: Mutex value missing on free fast path (core corrupt)"),
+                };
                 return Ok(MutexGuard { mutex: self, value: Some(value) });
             }
         }
@@ -135,8 +253,11 @@ impl<T> Mutex<T> {
         // Slow path: register as a waiter, set timeout, park.
         let _np = scheduler::NoPreempt::enter();
         let epoch = {
-            let mut st = self.core.state.lock().unwrap();
-            // begin_wait is lock-free — legal under the state lock; this
+            let mut st = match self.core.state.lock() {
+                Ok(g) => g,
+                Err(e) => panic!("smarm: mutex state lock poisoned (core corrupt): {e}"),
+            };
+            // begin_wait is lock-free (legal under the state lock); this
             // makes the epoch atomic with the registration's visibility to
             // grants and timeouts.
             let epoch = scheduler::begin_wait();
@@ -149,30 +270,51 @@ impl<T> Mutex<T> {
         scheduler::insert_wait_timer(deadline, me, target, epoch);
         scheduler::park_current();
 
-        // Resumed — precisely: only our grant or our timer can wake this
+        // Resumed, precisely: only our grant or our timer can wake this
         // wait (both epoch-stamped; a stop wake unwinds out of
         // park_current). The one-shot interpretation below is therefore
         // exhaustive. Are we the holder?
-        let is_holder = self.core.state.lock().unwrap().holder == Some(me);
+        let is_holder = match self.core.state.lock() {
+            Ok(st) => st.holder == Some(me),
+            Err(e) => panic!("smarm: mutex state lock poisoned (core corrupt): {e}"),
+        };
         if is_holder {
-            let value = self.value.lock().unwrap().take()
-                .expect("Mutex: value missing after grant");
+            let taken = match self.value.lock() {
+                Ok(mut g) => g.take(),
+                Err(e) => panic!("smarm: mutex value lock poisoned (core corrupt): {e}"),
+            };
+            let value = match taken {
+                Some(v) => v,
+                None => panic!("smarm: Mutex value missing after grant (core corrupt)"),
+            };
             Ok(MutexGuard { mutex: self, value: Some(value) })
         } else {
             Err(LockTimeout)
         }
     }
 
+    /// Acquire the lock only if it is immediately available: never parks and
+    /// never waits. Returns `Some` with a [`MutexGuard`] if the lock was
+    /// free, `None` if it is currently held elsewhere.
     pub fn try_lock(&self) -> Option<MutexGuard<'_, T>> {
         let me = crate::actor::current_pid()?;
-        let mut st = self.core.state.lock().unwrap();
+        let mut st = match self.core.state.lock() {
+            Ok(g) => g,
+            Err(e) => panic!("smarm: mutex state lock poisoned (core corrupt): {e}"),
+        };
         if st.holder.is_some() {
             return None;
         }
         st.holder = Some(me);
         drop(st);
-        let value = self.value.lock().unwrap().take()
-            .expect("Mutex: value missing on try_lock free path");
+        let taken = match self.value.lock() {
+            Ok(mut g) => g.take(),
+            Err(e) => panic!("smarm: mutex value lock poisoned (core corrupt): {e}"),
+        };
+        let value = match taken {
+            Some(v) => v,
+            None => panic!("smarm: Mutex value missing on try_lock free path (core corrupt)"),
+        };
         Some(MutexGuard { mutex: self, value: Some(value) })
     }
 
@@ -183,7 +325,10 @@ impl<T> Mutex<T> {
         // tracking and just grab the value mutex directly.  This is safe because
         // outside the runtime there are no green threads competing.
         let value = loop {
-            let v = self.value.lock().unwrap().take();
+            let v = match self.value.lock() {
+                Ok(mut g) => g.take(),
+                Err(e) => panic!("smarm: mutex value lock poisoned (core corrupt): {e}"),
+            };
             if let Some(v) = v { break v; }
             std::thread::yield_now();
         };
@@ -192,6 +337,10 @@ impl<T> Mutex<T> {
 }
 
 impl<T> Clone for Mutex<T> {
+    /// Cheap: hands back another handle to the same underlying lock and
+    /// value, the way `Arc::clone` does. All clones of a `Mutex` share one
+    /// lock and one protected value; locking through any clone excludes
+    /// every other clone.
     fn clone(&self) -> Self {
         Self { core: self.core.clone(), value: self.value.clone() }
     }
@@ -205,6 +354,10 @@ unsafe impl<T: Send> Sync for Mutex<T> {}
 // Guard
 // ---------------------------------------------------------------------------
 
+/// Grants access to the value inside a [`Mutex`] while the lock is held.
+/// Dereferences to `&T` and `&mut T`. Dropping the guard releases the lock
+/// and, if another actor is waiting, wakes the next one in arrival order.
+/// Returned by [`Mutex::lock`], [`Mutex::lock_timeout`], and [`Mutex::try_lock`].
 pub struct MutexGuard<'a, T> {
     mutex: &'a Mutex<T>,
     value: Option<T>,
@@ -212,30 +365,51 @@ pub struct MutexGuard<'a, T> {
 
 impl<T> std::ops::Deref for MutexGuard<'_, T> {
     type Target = T;
-    fn deref(&self) -> &T { self.value.as_ref().expect("MutexGuard: value missing") }
+    fn deref(&self) -> &T {
+        match self.value.as_ref() {
+            Some(v) => v,
+            None => panic!("smarm: MutexGuard value missing (core corrupt)"),
+        }
+    }
 }
 
 impl<T> std::ops::DerefMut for MutexGuard<'_, T> {
     fn deref_mut(&mut self) -> &mut T {
-        self.value.as_mut().expect("MutexGuard: value missing")
+        match self.value.as_mut() {
+            Some(v) => v,
+            None => panic!("smarm: MutexGuard value missing (core corrupt)"),
+        }
     }
 }
 
 impl<T: std::fmt::Debug> std::fmt::Debug for MutexGuard<'_, T> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let value = match self.value.as_ref() {
+            Some(v) => v,
+            None => panic!("smarm: MutexGuard value missing (core corrupt)"),
+        };
         f.debug_tuple("MutexGuard")
-            .field(self.value.as_ref().expect("MutexGuard: value missing"))
+            .field(value)
             .finish()
     }
 }
 
 impl<T> Drop for MutexGuard<'_, T> {
     fn drop(&mut self) {
-        let v = self.value.take().expect("MutexGuard: double drop");
-        *self.mutex.value.lock().unwrap() = Some(v);
+        let v = match self.value.take() {
+            Some(v) => v,
+            None => panic!("smarm: MutexGuard double drop (core corrupt)"),
+        };
+        match self.mutex.value.lock() {
+            Ok(mut g) => *g = Some(v),
+            Err(e) => panic!("smarm: mutex value lock poisoned (core corrupt): {e}"),
+        }
 
         let next = {
-            let mut st = self.mutex.core.state.lock().unwrap();
+            let mut st = match self.mutex.core.state.lock() {
+                Ok(g) => g,
+                Err(e) => panic!("smarm: mutex state lock poisoned (core corrupt): {e}"),
+            };
             match st.waiters.pop_front() {
                 Some(w) => {
                     st.holder = Some(w.pid);

src/park.rs

@@ -0,0 +1,994 @@
+//! Scheduler park/wake coordination layer (RFC 018).
+//!
+//! Schedulers never touch an fd to sleep: they park on a per-thread
+//! [`Parker`] and are woken through an idle-mask protocol the runtime owns
+//! outright. IO backends (epoll today, io_uring later) are *producers*
+//! behind a two-call contract — make actors runnable, then wake — which is
+//! what makes backend selection tractable (RFC 018 §step-back).
+//!
+//! Three pieces, all in [`Coordinator`]:
+//!
+//! - **Parker** (one per scheduler): permit semantics, `std::thread::park`
+//!   shaped — an unpark delivered before the park sets a permit; the next
+//!   park consumes it and returns immediately. This single property closes
+//!   the check-then-park race. Linux: `futex(2)` `FUTEX_WAIT`/`FUTEX_WAKE`
+//!   (private) with a nanosecond-precision relative `timespec` — the
+//!   `as_millis` truncation defect of the retired wake pipe is
+//!   unrepresentable here. Loom / non-Linux: `Mutex<bool>` + `Condvar`
+//!   (the loom models run against this build).
+//! - **Idle mask**: an `AtomicU64` bitmask of parked scheduler ids
+//!   (construction asserts ≤ 64 schedulers). Park protocol: set own bit,
+//!   run the caller's mandatory post-publish re-check, then wait. A
+//!   producer that published work before observing our bit has left us
+//!   work the re-check finds; one that observes the bit wakes us.
+//!
+//!   The publish/re-check pair is a store-buffer (Dekker) shape. Two sound
+//!   resolutions coexist here, chosen per call-site cost profile: the
+//!   **fence handshake** on the hot producer path
+//!   ([`Coordinator::wake_one_if_idle`]: publish work; `fence(SeqCst)`;
+//!   one *Relaxed* mask load — pairing with the consumer's `fetch_or(bit)`;
+//!   `fence(SeqCst)`; re-check inside [`Coordinator::park`]), so the
+//!   pure-compute hot path (everyone busy, mask 0) never takes the shared
+//!   mask line exclusive — the RFC's "one relaxed load" fast path, made
+//!   sound; and the **same-location-RMW read** (`fetch_or(0)`, in
+//!   [`Coordinator::wake_one`] / [`Coordinator::idle_mask`]) for the rare
+//!   paths (chain rule — at most one per wake) where reading the latest
+//!   mask by modification-order coherence is worth an RMW. The loom models
+//!   drive the fence pattern end to end (a fence-less plain-load draft
+//!   would — and did — fail model 1/2 with a lost wake, as it must).
+//! - **Timekeeper**: at most one parked scheduler holds the timer
+//!   deadline (RFC 018 §timers) so a timer expiry wakes one scheduler,
+//!   not a herd. The role is an atomic `(holder id, armed deadline)`
+//!   pair; a timer insertion with an earlier deadline wakes the holder to
+//!   re-peek. Arm / insert-check MUST be serialized by the caller (the
+//!   timers mutex in the runtime) — the atomics exist so the *busy-path
+//!   due-check* (one Relaxed load, [`Coordinator::armed_deadline_nanos`])
+//!   and the wake stay lock-free. All races are biased over-wake: a
+//!   spurious permit costs one failed pop; a missed wake would cost a
+//!   stranded actor, and is unrepresentable under the serialization rule.
+//!
+//! Every wake here is *at most one* futex round-trip and wakes *exactly
+//! one* scheduler by construction (`wake_one` CASes a bit clear before
+//! unparking its owner) — there is no shared level-triggered anything
+//! left to herd on.
+//!
+//! Standalone until the runtime swap (RFC 018 commit 2): nothing outside
+//! tests constructs a [`Coordinator`] yet.
+
+use crate::sync_shim::{fence, AtomicU64, Ordering};
+use std::time::Instant;
+
+/// Sentinel for "no timekeeper" in the holder word.
+const NO_TIMEKEEPER: u64 = u64::MAX;
+/// Sentinel for "no armed deadline" in the deadline word. Also what the
+/// busy-path due-check compares against: `now_nanos < armed` is one branch.
+pub(crate) const NO_DEADLINE: u64 = u64::MAX;
+
+/// Outcome of a [`Coordinator::park`] call.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) enum ParkResult {
+    /// A permit was consumed (wake delivered before or during the park).
+    Woken,
+    /// The deadline passed with no wake. Only possible when a deadline was
+    /// supplied (and never under loom, which has no time — see `park`).
+    TimedOut,
+    /// The post-publish re-check found work; the thread never blocked.
+    /// A permit may still be pending (a racing `wake_one` picked us after
+    /// the bit was set) — it will surface as one spurious `Woken` on a
+    /// later park. Benign: over-wake by design.
+    WorkFound,
+}
+
+// ---------------------------------------------------------------------------
+// Parker — permit-semantics thread parking
+// ---------------------------------------------------------------------------
+
+/// Linux, non-loom: futex on a state word.
+///
+/// States: EMPTY (no permit, nobody waiting), PARKED (a thread is, or is
+/// about to be, in `futex_wait`), NOTIFIED (permit pending). The classic
+/// std-parker protocol: `unpark` swaps to NOTIFIED and futex-wakes iff it
+/// displaced PARKED; `park` CASes EMPTY→PARKED, waits, and consumes
+/// NOTIFIED on every exit path.
+#[cfg(all(target_os = "linux", not(loom)))]
+mod parker {
+    use std::sync::atomic::{AtomicU32, Ordering};
+    use std::time::Instant;
+
+    const EMPTY: u32 = 0;
+    const PARKED: u32 = 1;
+    const NOTIFIED: u32 = 2;
+
+    pub(super) struct Parker {
+        state: AtomicU32,
+    }
+
+    impl Parker {
+        pub(super) fn new() -> Self {
+            Self { state: AtomicU32::new(EMPTY) }
+        }
+
+        /// Returns `true` = woken (permit consumed), `false` = timed out.
+        pub(super) fn park(&self, deadline: Option<Instant>) -> bool {
+            // Fast path: consume a pending permit without blocking.
+            if self
+                .state
+                .compare_exchange(EMPTY, PARKED, Ordering::AcqRel, Ordering::Acquire)
+                .is_err()
+            {
+                // Only NOTIFIED can be here (one thread parks at a time).
+                self.state.store(EMPTY, Ordering::Release);
+                return true;
+            }
+            loop {
+                let timeout = match deadline {
+                    None => None,
+                    Some(d) => {
+                        let now = Instant::now();
+                        if d <= now {
+                            // Deadline passed: cancel the park. The swap
+                            // races a concurrent unpark — if it delivered
+                            // NOTIFIED first, report Woken (never lose a
+                            // permit).
+                            return self.state.swap(EMPTY, Ordering::AcqRel) == NOTIFIED;
+                        }
+                        Some(d - now)
+                    }
+                };
+                futex_wait(&self.state, PARKED, timeout);
+                if self
+                    .state
+                    .compare_exchange(NOTIFIED, EMPTY, Ordering::AcqRel, Ordering::Acquire)
+                    .is_ok()
+                {
+                    return true;
+                }
+                // Spurious wake or timeout with state still PARKED: loop —
+                // the deadline check at the top decides.
+            }
+        }
+
+        pub(super) fn unpark(&self) {
+            if self.state.swap(NOTIFIED, Ordering::AcqRel) == PARKED {
+                futex_wake(&self.state, 1);
+            }
+        }
+    }
+
+    /// `FUTEX_WAIT` with a *relative* nanosecond timeout (`CLOCK_MONOTONIC`
+    /// per futex(2) for relative waits). No millisecond conversion anywhere:
+    /// the timespec carries the full sub-ms remainder (RFC 018 kills the
+    /// `as_millis` truncation structurally).
+    fn futex_wait(word: &AtomicU32, expected: u32, timeout: Option<std::time::Duration>) {
+        let ts;
+        let ts_ptr: *const libc::timespec = match timeout {
+            Some(d) => {
+                ts = libc::timespec {
+                    tv_sec: d.as_secs() as libc::time_t,
+                    tv_nsec: d.subsec_nanos() as libc::c_long,
+                };
+                &ts
+            }
+            None => std::ptr::null(),
+        };
+        // Errors (EAGAIN: word changed; ETIMEDOUT; EINTR) all mean "return
+        // and let the caller's state machine decide" — deliberately ignored.
+        unsafe {
+            libc::syscall(
+                libc::SYS_futex,
+                word.as_ptr(),
+                libc::FUTEX_WAIT | libc::FUTEX_PRIVATE_FLAG,
+                expected,
+                ts_ptr,
+            );
+        }
+    }
+
+    fn futex_wake(word: &AtomicU32, n: u32) {
+        unsafe {
+            libc::syscall(
+                libc::SYS_futex,
+                word.as_ptr(),
+                libc::FUTEX_WAKE | libc::FUTEX_PRIVATE_FLAG,
+                n,
+            );
+        }
+    }
+}
+
+/// Loom / non-Linux: `Mutex<bool>` permit + `Condvar` — loom's own model
+/// of a parker, and the portable fallback. Under loom the deadline is
+/// ignored (loom has no time); the models exercise wake paths only.
+#[cfg(any(loom, not(target_os = "linux")))]
+mod parker {
+    use crate::sync_shim::{Condvar, Mutex};
+    use std::time::Instant;
+
+    pub(super) struct Parker {
+        permit: Mutex<bool>,
+        cv: Condvar,
+    }
+
+    impl Parker {
+        pub(super) fn new() -> Self {
+            Self { permit: Mutex::new(false), cv: Condvar::new() }
+        }
+
+        /// Returns `true` = woken (permit consumed), `false` = timed out.
+        pub(super) fn park(&self, deadline: Option<Instant>) -> bool {
+            let mut permit = match self.permit.lock() {
+                Ok(g) => g,
+                Err(_) => panic!("smarm: parker permit lock poisoned (core corrupt)"),
+            };
+            loop {
+                if *permit {
+                    *permit = false;
+                    return true;
+                }
+                #[cfg(loom)]
+                {
+                    // Loom has no clock: block until a wake. Models must
+                    // deliver one (a park nobody wakes is a real deadlock
+                    // and loom reports it as such).
+                    let _ = deadline;
+                    permit = match self.cv.wait(permit) {
+                        Ok(g) => g,
+                        Err(_) => panic!("smarm: parker cv poisoned (core corrupt)"),
+                    };
+                }
+                #[cfg(not(loom))]
+                {
+                    match deadline {
+                        None => {
+                            permit = match self.cv.wait(permit) {
+                                Ok(g) => g,
+                                Err(_) => panic!("smarm: parker cv poisoned (core corrupt)"),
+                            };
+                        }
+                        Some(d) => {
+                            let now = Instant::now();
+                            if d <= now {
+                                return false;
+                            }
+                            permit = match self.cv.wait_timeout(permit, d - now) {
+                                Ok((g, _)) => g,
+                                Err(_) => panic!("smarm: parker cv poisoned (core corrupt)"),
+                            };
+                        }
+                    }
+                }
+            }
+        }
+
+        pub(super) fn unpark(&self) {
+            let mut permit = match self.permit.lock() {
+                Ok(g) => g,
+                Err(_) => panic!("smarm: parker permit lock poisoned (core corrupt)"),
+            };
+            *permit = true;
+            self.cv.notify_one();
+        }
+    }
+}
+
+use parker::Parker;
+
+// ---------------------------------------------------------------------------
+// Coordinator — idle mask + wake protocol + timekeeper
+// ---------------------------------------------------------------------------
+
+pub(crate) struct Coordinator {
+    parkers: Box<[Parker]>,
+    /// Bit `i` set = scheduler `i` is parked or committed to parking (set
+    /// before the re-check; cleared by `wake_one`'s CAS or by the parker
+    /// itself on return). AcqRel same-location-RMW handshake — see module
+    /// docs (no SeqCst needed: every producer-side read is an RMW).
+    idle: AtomicU64,
+    /// Timekeeper holder id, or `NO_TIMEKEEPER`. Written under the
+    /// caller's timer serialization (arm/disarm/insert-check); read
+    /// lock-free by the insert wake path.
+    tk_holder: AtomicU64,
+    /// Armed deadline as nanos since `origin`, or `NO_DEADLINE`. Written
+    /// only by the timekeeper arm/disarm protocol.
+    tk_armed: AtomicU64,
+    /// Earliest KNOWN timer deadline (nanos since `origin`), or
+    /// `NO_DEADLINE` — independent of whether any scheduler is parked,
+    /// which is what the timekeeper's `tk_armed` cannot give: under
+    /// saturation nobody parks and nobody arms, yet due timers must still
+    /// fire (ratified design point (a)). Maintained under the caller's
+    /// timers mutex (`note_deadline` on insert, `refresh_deadline` after a
+    /// pop/peek); read lock-free by the busy-path due-check.
+    next_deadline: AtomicU64,
+    /// Time origin for the nanos encoding.
+    origin: Instant,
+}
+
+impl Coordinator {
+    pub(crate) fn new(schedulers: usize) -> Self {
+        assert!(
+            (1..=64).contains(&schedulers),
+            "smarm: scheduler count must be 1..=64 (idle mask is one u64); got {schedulers}"
+        );
+        Self {
+            parkers: (0..schedulers).map(|_| Parker::new()).collect(),
+            idle: AtomicU64::new(0),
+            tk_holder: AtomicU64::new(NO_TIMEKEEPER),
+            tk_armed: AtomicU64::new(NO_DEADLINE),
+            next_deadline: AtomicU64::new(NO_DEADLINE),
+            origin: Instant::now(),
+        }
+    }
+
+    /// Encode a deadline for the armed snapshot / busy-path compare.
+    /// Saturating: a deadline at-or-before `origin` encodes as 0 (always
+    /// due), one beyond ~584 years as `NO_DEADLINE - 1`.
+    pub(crate) fn deadline_nanos(&self, deadline: Instant) -> u64 {
+        let nanos = deadline
+            .checked_duration_since(self.origin)
+            .map(|d| d.as_nanos())
+            .unwrap_or(0);
+        if nanos >= NO_DEADLINE as u128 {
+            NO_DEADLINE - 1
+        } else {
+            nanos as u64
+        }
+    }
+
+    /// Park scheduler `id` until a wake, the deadline, or a positive
+    /// re-check. Protocol: (1) publish own idle bit (SeqCst), (2) run
+    /// `recheck` — it MUST re-read the work source with an ordering that
+    /// pairs with the producer's publish (SeqCst load, or take the mutex
+    /// the producer publishes under); a `true` aborts the park, (3) block.
+    pub(crate) fn park(
+        &self,
+        id: usize,
+        deadline: Option<Instant>,
+        recheck: impl FnOnce() -> bool,
+    ) -> ParkResult {
+        debug_assert!(id < self.parkers.len(), "park: scheduler id out of range");
+        let bit = 1u64 << id;
+        // (1) publish. AcqRel RMW: the acquire half is the handshake — if
+        // this lands after a producer's mask RMW in modification order, we
+        // read-from it and the producer's earlier work publication is
+        // visible to the re-check below (see module docs).
+        let prev = self.idle.fetch_or(bit, Ordering::AcqRel);
+        debug_assert_eq!(prev & bit, 0, "park: idle bit already set for this id");
+        // Fence half of the producer handshake (see `wake_one_if_idle`):
+        // orders our bit-publish before the re-check's loads, so it pairs
+        // with the producer's publish→fence→mask-load — at least one side
+        // must see the other's store, whichever queue backend is in play.
+        fence(Ordering::SeqCst);
+        // (2) the mandatory post-publish re-check.
+        if recheck() {
+            self.idle.fetch_and(!bit, Ordering::AcqRel);
+            return ParkResult::WorkFound;
+        }
+        // (3) block. The permit closes the window between the re-check and
+        // the futex wait: a wake_one that picked us in that window has
+        // already CASed our bit clear and set the permit.
+        let woken = self.parkers[id].park(deadline);
+        // Clear own bit — a no-op when a waker already CASed it clear.
+        self.idle.fetch_and(!bit, Ordering::AcqRel);
+        if woken {
+            ParkResult::Woken
+        } else {
+            ParkResult::TimedOut
+        }
+    }
+
+    /// Wake exactly one parked scheduler, if any: pick the highest set idle
+    /// bit (LIFO-ish — warmest cache), CAS it clear, deliver a permit.
+    /// Empty mask = no-op (everyone is awake and will find work by
+    /// popping). Returns whether a scheduler was woken.
+    pub(crate) fn wake_one(&self) -> bool {
+        // RMW read, not a load: reads the latest mask by modification-order
+        // coherence, closing the Dekker race with a parking consumer (see
+        // module docs). The release side of the RMW is what a later-parking
+        // consumer's fetch_or acquires to make its re-check sound.
+        let mut mask = self.idle.fetch_or(0, Ordering::AcqRel);
+        loop {
+            if mask == 0 {
+                return false;
+            }
+            let id = 63 - mask.leading_zeros() as usize; // highest set bit
+            let bit = 1u64 << id;
+            // The CAS is the exactly-one guarantee: whoever clears the bit
+            // owns the wake; a racing wake_one retries on the observed value
+            // (coherence: a failed CAS can never read older than `mask`).
+            match self.idle.compare_exchange(
+                mask,
+                mask & !bit,
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            ) {
+                Ok(_) => {
+                    self.parkers[id].unpark();
+                    return true;
+                }
+                Err(m) => mask = m,
+            }
+        }
+    }
+
+    /// The producer-side wake tail (`enqueue`'s fast path, RFC 018 "enqueue
+    /// wakes"). The caller has just published work (queue push); we fence,
+    /// then read the mask with ONE Relaxed load — 0 means every scheduler
+    /// is awake and the pure-compute hot path pays no RMW on the shared
+    /// mask line. Soundness is the fence handshake (module docs): our
+    /// fence orders the caller's push before the mask load; the consumer's
+    /// fence (in `park`) orders its bit-publish before its re-check — at
+    /// least one side must observe the other's store, so a consumer we
+    /// miss here is a consumer whose re-check finds the caller's work.
+    pub(crate) fn wake_one_if_idle(&self) -> bool {
+        fence(Ordering::SeqCst);
+        if self.idle.load(Ordering::Relaxed) == 0 {
+            return false;
+        }
+        self.wake_one()
+    }
+
+    /// Terminal wake (replaces the AllDone wake-pipe byte): clear the mask
+    /// and deliver a permit to *every* parker, parked or not. A permit set
+    /// on a busy scheduler costs one spurious park return — nothing at the
+    /// terminal boundary. Idempotent.
+    pub(crate) fn wake_all(&self) {
+        self.idle.store(0, Ordering::Release);
+        for p in self.parkers.iter() {
+            p.unpark();
+        }
+    }
+
+    /// Latest idle mask (RMW read — same handshake as `wake_one`). A
+    /// test-only observer: production expresses the chain rule through
+    /// `wake_one_if_idle` (fence + Relaxed load), not a mask read.
+    #[cfg(test)]
+    pub(crate) fn idle_mask(&self) -> u64 {
+        self.idle.fetch_or(0, Ordering::AcqRel)
+    }
+
+    // ----- timekeeper -----
+
+    /// Try to take the timekeeper role for scheduler `id` with `deadline`.
+    /// MUST be called under the caller's timer serialization (the timers
+    /// mutex), with `deadline` the heap minimum peeked under that same
+    /// hold — this is what makes the insert-check race-free. Returns
+    /// whether the role was taken (false = someone else holds it; park
+    /// with no deadline).
+    pub(crate) fn try_arm_timer(&self, id: usize, deadline: Instant) -> bool {
+        debug_assert!(id < self.parkers.len(), "try_arm_timer: id out of range");
+        if self
+            .tk_holder
+            .compare_exchange(NO_TIMEKEEPER, id as u64, Ordering::SeqCst, Ordering::SeqCst)
+            .is_err()
+        {
+            return false;
+        }
+        // Holder-then-deadline order: an insert-check that sees the holder
+        // with the deadline still NO_DEADLINE compares `new < MAX` = true
+        // and over-wakes — the benign direction. (Under the mandated timer
+        // serialization this interleaving cannot occur anyway.)
+        self.tk_armed.store(self.deadline_nanos(deadline), Ordering::SeqCst);
+        true
+    }
+
+    /// Release the timekeeper role (the holder, on wake, before it
+    /// re-peeks/fires). Callable without the timer serialization: a
+    /// racing insert may wake a no-longer-holder — over-wake, benign.
+    pub(crate) fn disarm_timer(&self, id: usize) {
+        debug_assert_eq!(
+            self.tk_holder.load(Ordering::SeqCst),
+            id as u64,
+            "disarm_timer by a non-holder"
+        );
+        // Deadline first: a concurrent insert-check then sees NO_DEADLINE
+        // and skips the (now pointless) wake instead of waking a stale
+        // holder id. Either order is correct; this one wastes less.
+        self.tk_armed.store(NO_DEADLINE, Ordering::SeqCst);
+        self.tk_holder.store(NO_TIMEKEEPER, Ordering::SeqCst);
+    }
+
+    /// Insert-side re-arm check: if `deadline` is earlier than the armed
+    /// snapshot, wake the timekeeper to re-peek. MUST be called under the
+    /// same timer serialization as `try_arm_timer` (see there).
+    pub(crate) fn timer_inserted(&self, deadline: Instant) {
+        if self.deadline_nanos(deadline) < self.tk_armed.load(Ordering::SeqCst) {
+            let holder = self.tk_holder.load(Ordering::SeqCst);
+            if holder != NO_TIMEKEEPER {
+                // Direct unpark, not wake_one: the wake targets the
+                // timekeeper specifically (it must re-peek the heap). Its
+                // idle bit stays set until it returns from park — a
+                // concurrent wake_one may pick it too; over-wake, benign.
+                self.parkers[holder as usize].unpark();
+            }
+        }
+    }
+
+    /// The armed-deadline snapshot (nanos since origin; `NO_DEADLINE` =
+    /// none). Test-only introspection on the timekeeper's armed value; the
+    /// busy-path due-check reads `next_deadline`, not this.
+    #[cfg(test)]
+    pub(crate) fn armed_deadline_nanos(&self) -> u64 {
+        self.tk_armed.load(Ordering::Relaxed)
+    }
+
+    // ----- earliest-deadline snapshot (busy-path due-check) -----
+
+    /// Record a newly inserted timer deadline. MUST be called under the
+    /// timers mutex (same serialization rule as `try_arm_timer`), which is
+    /// why plain compare+store suffices for the min-maintenance. Also runs
+    /// the timekeeper re-arm check (`timer_inserted`) — one call site for
+    /// both consequences of an insert.
+    pub(crate) fn note_deadline(&self, deadline: Instant) {
+        let n = self.deadline_nanos(deadline);
+        if n < self.next_deadline.load(Ordering::Relaxed) {
+            self.next_deadline.store(n, Ordering::Release);
+        }
+        self.timer_inserted(deadline);
+    }
+
+    /// Re-anchor the snapshot to the heap minimum (`None` = heap empty)
+    /// after a `pop_due` / `clear`. MUST be called under the timers mutex.
+    pub(crate) fn refresh_deadline(&self, next: Option<Instant>) {
+        let n = next.map_or(NO_DEADLINE, |d| self.deadline_nanos(d));
+        self.next_deadline.store(n, Ordering::Release);
+    }
+
+    /// Busy-path due-check: is the earliest known deadline at or past
+    /// `now`? One Relaxed load when no deadline is armed — the clock is
+    /// read only when a timer actually exists (matching the old drain
+    /// phase's is_empty guard), so the pure-compute hot path pays a load
+    /// and a branch.
+    pub(crate) fn deadline_due(&self) -> bool {
+        let n = self.next_deadline.load(Ordering::Relaxed);
+        n != NO_DEADLINE && self.deadline_nanos(Instant::now()) >= n
+    }
+
+    /// The earliest-deadline snapshot as an `Instant` (`None` = no timer
+    /// pending). Test-only: the idle path arms the timekeeper from the
+    /// timer heap's own `peek_deadline` under the timers mutex.
+    #[cfg(test)]
+    pub(crate) fn next_deadline_instant(&self) -> Option<Instant> {
+        let n = self.next_deadline.load(Ordering::Acquire);
+        if n == NO_DEADLINE {
+            None
+        } else {
+            self.origin.checked_add(std::time::Duration::from_nanos(n))
+        }
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Unit tests (std build)
+// ---------------------------------------------------------------------------
+
+#[cfg(all(test, not(loom)))]
+mod tests {
+    use super::*;
+    use std::sync::atomic::{AtomicUsize, Ordering as O};
+    use std::sync::Arc;
+    use std::time::{Duration, Instant};
+
+    #[test]
+    fn permit_before_park_returns_immediately() {
+        let c = Coordinator::new(1);
+        // Deliver the wake first (nobody parked: wake_one no-ops on the
+        // mask, so use the timekeeper-direct path? No — permit semantics
+        // are the parker's own; exercise via wake_all which permits all).
+        c.wake_all();
+        let t0 = Instant::now();
+        let r = c.park(0, None, || false);
+        assert_eq!(r, ParkResult::Woken);
+        assert!(t0.elapsed() < Duration::from_millis(100), "park blocked despite permit");
+    }
+
+    #[test]
+    fn submillisecond_deadline_is_honored() {
+        // Regression for the retired as_millis truncation: a 500µs deadline
+        // must neither busy-return instantly forever nor round to 0/∞.
+        let c = Coordinator::new(1);
+        let t0 = Instant::now();
+        let r = c.park(0, Some(t0 + Duration::from_micros(500)), || false);
+        let dt = t0.elapsed();
+        assert_eq!(r, ParkResult::TimedOut);
+        assert!(dt >= Duration::from_micros(400), "woke too early: {dt:?}");
+        assert!(dt < Duration::from_millis(50), "overslept: {dt:?}");
+    }
+
+    #[test]
+    fn recheck_true_aborts_park_and_clears_bit() {
+        let c = Coordinator::new(2);
+        let r = c.park(1, None, || true);
+        assert_eq!(r, ParkResult::WorkFound);
+        assert_eq!(c.idle_mask(), 0, "bit not cleared after WorkFound");
+    }
+
+    #[test]
+    fn recheck_observes_own_bit_published() {
+        let c = Coordinator::new(2);
+        let seen = std::cell::Cell::new(0u64);
+        let r = c.park(1, None, || {
+            seen.set(c.idle_mask());
+            true
+        });
+        assert_eq!(r, ParkResult::WorkFound);
+        assert_eq!(seen.get() & 0b10, 0b10, "bit not published before re-check");
+    }
+
+    #[test]
+    fn wake_one_wakes_exactly_one_of_n() {
+        const N: usize = 4;
+        let c = Arc::new(Coordinator::new(N));
+        let woken = Arc::new(AtomicUsize::new(0));
+        let mut ts = Vec::new();
+        for id in 0..N {
+            let c = c.clone();
+            let woken = woken.clone();
+            ts.push(std::thread::spawn(move || {
+                let r = c.park(id, None, || false);
+                assert_eq!(r, ParkResult::Woken);
+                woken.fetch_add(1, O::SeqCst);
+            }));
+        }
+        // Wait until all four are published idle.
+        let t0 = Instant::now();
+        while c.idle_mask().count_ones() != N as u32 {
+            assert!(t0.elapsed() < Duration::from_secs(5), "threads never parked");
+            std::thread::yield_now();
+        }
+        assert!(c.wake_one());
+        // Exactly one wakes; give the others a beat to (incorrectly) wake.
+        let t0 = Instant::now();
+        while woken.load(O::SeqCst) == 0 {
+            assert!(t0.elapsed() < Duration::from_secs(5), "wake_one woke nobody");
+            std::thread::yield_now();
+        }
+        std::thread::sleep(Duration::from_millis(100));
+        assert_eq!(woken.load(O::SeqCst), 1, "wake_one woke more than one");
+        assert_eq!(c.idle_mask().count_ones(), (N - 1) as u32);
+        c.wake_all();
+        for t in ts {
+            match t.join() {
+                Ok(()) => {}
+                Err(p) => std::panic::resume_unwind(p),
+            }
+        }
+        assert_eq!(woken.load(O::SeqCst), N);
+        assert_eq!(c.idle_mask(), 0);
+    }
+
+    #[test]
+    fn wake_one_prefers_highest_bit() {
+        let c = Arc::new(Coordinator::new(3));
+        let woken_id = Arc::new(AtomicUsize::new(usize::MAX));
+        let mut ts = Vec::new();
+        for id in 0..3 {
+            let c = c.clone();
+            let woken_id = woken_id.clone();
+            ts.push(std::thread::spawn(move || {
+                if c.park(id, None, || false) == ParkResult::Woken {
+                    let _ = woken_id.compare_exchange(usize::MAX, id, O::SeqCst, O::SeqCst);
+                }
+            }));
+        }
+        let t0 = Instant::now();
+        while c.idle_mask() != 0b111 {
+            assert!(t0.elapsed() < Duration::from_secs(5));
+            std::thread::yield_now();
+        }
+        assert!(c.wake_one());
+        let t0 = Instant::now();
+        while woken_id.load(O::SeqCst) == usize::MAX {
+            assert!(t0.elapsed() < Duration::from_secs(5));
+            std::thread::yield_now();
+        }
+        assert_eq!(woken_id.load(O::SeqCst), 2, "LIFO-ish: highest bit first");
+        c.wake_all();
+        for t in ts {
+            let _ = t.join();
+        }
+    }
+
+    #[test]
+    fn wake_one_on_empty_mask_is_noop() {
+        let c = Coordinator::new(2);
+        assert!(!c.wake_one());
+        assert_eq!(c.idle_mask(), 0);
+    }
+
+    #[test]
+    fn timekeeper_arm_is_exclusive_and_snapshot_readable() {
+        let c = Coordinator::new(2);
+        let d2 = Instant::now() + Duration::from_secs(10);
+        let d1 = Instant::now() + Duration::from_secs(1);
+        assert_eq!(c.armed_deadline_nanos(), NO_DEADLINE);
+        assert!(c.try_arm_timer(0, d2));
+        assert!(!c.try_arm_timer(1, d1), "second arm must fail while held");
+        assert_eq!(c.armed_deadline_nanos(), c.deadline_nanos(d2));
+        c.disarm_timer(0);
+        assert_eq!(c.armed_deadline_nanos(), NO_DEADLINE);
+        assert!(c.try_arm_timer(1, d1), "role must be re-takeable after disarm");
+        c.disarm_timer(1);
+    }
+
+    #[test]
+    fn earlier_insert_wakes_timekeeper() {
+        let c = Coordinator::new(2);
+        let far = Instant::now() + Duration::from_secs(60);
+        let near = Instant::now() + Duration::from_millis(1);
+        assert!(c.try_arm_timer(0, far));
+        // Holder not yet parked: the wake must land as a permit.
+        c.timer_inserted(near);
+        let t0 = Instant::now();
+        let r = c.park(0, Some(far), || false);
+        assert_eq!(r, ParkResult::Woken, "re-arm wake lost");
+        assert!(t0.elapsed() < Duration::from_secs(5), "slept toward the stale deadline");
+        c.disarm_timer(0);
+    }
+
+    #[test]
+    fn later_insert_does_not_wake_timekeeper() {
+        let c = Coordinator::new(2);
+        let near = Instant::now() + Duration::from_millis(20);
+        let far = Instant::now() + Duration::from_secs(60);
+        assert!(c.try_arm_timer(0, near));
+        c.timer_inserted(far); // later than armed: no wake
+        let r = c.park(0, Some(near), || false);
+        assert_eq!(r, ParkResult::TimedOut, "spurious wake for a later insert");
+        c.disarm_timer(0);
+    }
+
+    #[test]
+    #[should_panic(expected = "1..=64")]
+    fn more_than_64_schedulers_asserts() {
+        let _ = Coordinator::new(65);
+    }
+
+    #[test]
+    fn wake_one_if_idle_noop_on_empty_and_wakes_on_parked() {
+        let c = Arc::new(Coordinator::new(1));
+        assert!(!c.wake_one_if_idle(), "empty mask must be a no-op");
+        let c2 = c.clone();
+        let t = std::thread::spawn(move || {
+            assert_eq!(c2.park(0, None, || false), ParkResult::Woken);
+        });
+        let t0 = Instant::now();
+        while c.idle_mask() == 0 {
+            assert!(t0.elapsed() < Duration::from_secs(5), "never parked");
+            std::thread::yield_now();
+        }
+        assert!(c.wake_one_if_idle());
+        match t.join() {
+            Ok(()) => {}
+            Err(p) => std::panic::resume_unwind(p),
+        }
+    }
+
+    #[test]
+    fn deadline_snapshot_min_maintenance_and_due_check() {
+        let c = Coordinator::new(1);
+        assert!(!c.deadline_due(), "no deadline: never due");
+        assert_eq!(c.next_deadline_instant(), None);
+        let far = Instant::now() + Duration::from_secs(60);
+        let near = Instant::now() + Duration::from_millis(1);
+        c.note_deadline(far);
+        assert!(!c.deadline_due());
+        c.note_deadline(near); // min wins
+        assert!(c.next_deadline_instant().is_some_and(|d| d <= near));
+        c.note_deadline(far); // later insert must NOT raise the snapshot
+        assert!(c.next_deadline_instant().is_some_and(|d| d <= near));
+        std::thread::sleep(Duration::from_millis(2));
+        assert!(c.deadline_due(), "past deadline not reported due");
+        c.refresh_deadline(Some(far));
+        assert!(!c.deadline_due(), "refresh did not re-anchor");
+        c.refresh_deadline(None);
+        assert_eq!(c.next_deadline_instant(), None);
+        // A deadline at/before origin encodes as 0: always due.
+        c.note_deadline(Instant::now() - Duration::from_secs(1));
+        assert!(c.deadline_due());
+    }
+
+    #[test]
+    fn note_deadline_earlier_wakes_timekeeper_via_snapshot_path() {
+        // note_deadline must carry the timer_inserted re-arm wake too.
+        let c = Coordinator::new(2);
+        let far = Instant::now() + Duration::from_secs(60);
+        let near = Instant::now() + Duration::from_millis(1);
+        assert!(c.try_arm_timer(0, far));
+        c.note_deadline(near);
+        let r = c.park(0, Some(far), || false);
+        assert_eq!(r, ParkResult::Woken, "re-arm wake lost through note_deadline");
+        c.disarm_timer(0);
+    }
+}
+
+// ---------------------------------------------------------------------------
+// loom models — RUSTFLAGS="--cfg loom" cargo test --lib --release park
+// ---------------------------------------------------------------------------
+
+#[cfg(all(test, loom))]
+mod loom_tests {
+    use super::*;
+    use loom::sync::atomic::{AtomicU64 as LAtomicU64, Ordering as O};
+    use loom::sync::{Arc, Mutex as LMutex};
+    use loom::thread;
+    use std::time::{Duration, Instant};
+
+    /// RFC 018 loom model 1 — no lost wake.
+    /// producer{publish item; wake_one} ∥ consumer{set bit; re-check; park}:
+    /// the consumer always observes the item or a permit; it can never
+    /// sleep past a published item (loom's deadlock detector is the
+    /// assertion — a consumer parked forever fails the model).
+    #[test]
+    fn no_lost_wake() {
+        loom::model(|| {
+            let c = Arc::new(Coordinator::new(1));
+            let item = Arc::new(LAtomicU64::new(0));
+
+            let prod = {
+                let c = c.clone();
+                let item = item.clone();
+                thread::spawn(move || {
+                    // The enqueue shape: publish, then the fenced fast-path
+                    // wake tail (this is what the runtime's enqueue calls).
+                    item.store(1, O::SeqCst);
+                    c.wake_one_if_idle();
+                })
+            };
+
+            // Consumer: loop until the item is popped. Guarded load+CAS,
+            // not a blind swap — a swap writes 0 even when empty, and
+            // coherence allows that write to land after the producer's
+            // store in modification order, destroying the item (a model
+            // bug loom caught in an earlier draft of this test).
+            loop {
+                if item.load(O::SeqCst) == 1
+                    && item.compare_exchange(1, 0, O::SeqCst, O::SeqCst).is_ok()
+                {
+                    break;
+                }
+                let _ = c.park(0, None, || item.load(O::SeqCst) == 1);
+            }
+            prod.join().unwrap();
+        });
+    }
+
+    /// RFC 018 loom model 2 — chain propagation.
+    /// Two items, two sleepers, ONE producer wake: the chain rule (a woken
+    /// consumer that sees surplus work and a non-empty mask wakes again)
+    /// must get both items consumed with no further producer action.
+    #[test]
+    fn chain_propagation() {
+        loom::model(|| {
+            let c = Arc::new(Coordinator::new(2));
+            let items = Arc::new(LAtomicU64::new(0));
+            let consumed = Arc::new(LAtomicU64::new(0));
+
+            let mut hs = Vec::new();
+            for id in 0..2usize {
+                let c = c.clone();
+                let items = items.clone();
+                let consumed = consumed.clone();
+                hs.push(thread::spawn(move || loop {
+                    if consumed.load(O::SeqCst) == 2 {
+                        c.wake_all(); // release a sibling still parked
+                        break;
+                    }
+                    let cur = items.load(O::SeqCst);
+                    if cur > 0
+                        && items
+                            .compare_exchange(cur, cur - 1, O::SeqCst, O::SeqCst)
+                            .is_ok()
+                    {
+                        consumed.fetch_add(1, O::SeqCst);
+                        // THE CHAIN RULE, exactly as production expresses it
+                        // (runtime.rs schedule_loop): surplus ⇒ the fenced
+                        // fast-path wake. Models the Relaxed-load chain path,
+                        // not just the RMW one.
+                        if items.load(O::SeqCst) > 0 {
+                            c.wake_one_if_idle();
+                        }
+                        continue;
+                    }
+                    let _ = c.park(id, None, || {
+                        items.load(O::SeqCst) > 0 || consumed.load(O::SeqCst) == 2
+                    });
+                }));
+            }
+
+            // Producer (main): two items, ONE wake, via the enqueue-shaped
+            // fenced fast path.
+            items.store(2, O::SeqCst);
+            c.wake_one_if_idle();
+
+            for h in hs {
+                h.join().unwrap();
+            }
+            assert_eq!(consumed.load(O::SeqCst), 2);
+            assert_eq!(items.load(O::SeqCst), 0);
+        });
+    }
+
+    /// RFC 018 loom model 3 — timekeeper handoff.
+    /// An earlier-deadline insert racing the parking timekeeper: the
+    /// earlier deadline is always honored — either the timekeeper armed it
+    /// directly (insert landed first under the timers lock) or the insert
+    /// wakes the timekeeper to re-peek. A timekeeper sleeping toward the
+    /// stale later deadline would deadlock the model (loom has no time).
+    #[test]
+    fn timekeeper_handoff() {
+        loom::model(|| {
+            let origin = Instant::now();
+            let d_far = origin + Duration::from_secs(60);
+            let d_near = origin + Duration::from_secs(1);
+
+            let c = Arc::new(Coordinator::new(1));
+            // The timers-mutex stand-in: heap min under a lock.
+            let heap_min = Arc::new(LMutex::new(d_far));
+
+            let tk = {
+                let c = c.clone();
+                let heap_min = heap_min.clone();
+                thread::spawn(move || {
+                    // Peek + arm under the lock (the serialization rule).
+                    let armed = {
+                        let g = heap_min.lock().unwrap();
+                        let min = *g;
+                        assert!(c.try_arm_timer(0, min));
+                        min
+                    };
+                    if armed == d_far {
+                        // Insert hadn't landed: it MUST wake us. Parking
+                        // toward d_far with no wake = model deadlock.
+                        let r = c.park(0, Some(armed), || false);
+                        assert_eq!(r, ParkResult::Woken, "re-arm wake lost");
+                    }
+                    // Woken (or armed the near deadline directly): re-peek.
+                    c.disarm_timer(0);
+                    let g = heap_min.lock().unwrap();
+                    assert_eq!(*g, d_near, "earlier deadline not visible on re-peek");
+                })
+            };
+
+            // Inserter (main): publish the earlier deadline under the lock,
+            // then the insert-check.
+            {
+                let mut g = heap_min.lock().unwrap();
+                *g = d_near;
+                c.timer_inserted(d_near);
+            }
+
+            tk.join().unwrap();
+        });
+    }
+
+    /// RFC 018 loom model 4 — termination.
+    /// The AllDone verdict: producer flips done and wake_all()s; consumers
+    /// must never park forever past done (park's re-check + wake_all's
+    /// permits close every interleaving; a stuck consumer = loom deadlock).
+    #[test]
+    fn termination_no_park_past_done() {
+        loom::model(|| {
+            let c = Arc::new(Coordinator::new(2));
+            let done = Arc::new(LAtomicU64::new(0));
+
+            let mut hs = Vec::new();
+            for id in 0..2usize {
+                let c = c.clone();
+                let done = done.clone();
+                hs.push(thread::spawn(move || loop {
+                    if done.load(O::SeqCst) == 1 {
+                        break;
+                    }
+                    let _ = c.park(id, None, || done.load(O::SeqCst) == 1);
+                }));
+            }
+
+            done.store(1, O::SeqCst);
+            c.wake_all();
+
+            for h in hs {
+                h.join().unwrap();
+            }
+        });
+    }
+}

src/pg.rs

@@ -1,15 +1,17 @@
-//! Process groups — a `name → multiset<Member>` map (RFC 012).
+//! Process groups: one name, many actors.
 //!
-//! Sits parallel to [`registry`](crate::registry), not on top of it. The
-//! registry is a *bimap*: at most one pid per name. A group is the opposite —
-//! many pids per name, the same pid in many groups — so it cannot be a
-//! generalization of the bimap; it is its own keyspace sharing only the
-//! liveness signal source (monitors) and the lock *class*.
+//! A process group is a named set of actors that you can look up, fan out to,
+//! or pick a worker from. It is the natural home for a *worker pool* (several
+//! interchangeable actors doing the same job), for *service discovery* (find
+//! everyone currently offering some capability), and for *broadcast* (reach
+//! every member of a group at once).
 //!
-//! ## Example
+//! The set is *live*: members [`join`] it, and a member that dies is removed
+//! automatically. You never deregister a dead actor — there is no bookkeeping
+//! to get wrong, and [`members`] / [`pick`] never hand you an actor that has
+//! already gone.
 //!
-//! A group is a live membership view: members join, and a member that dies is
-//! evicted automatically — no deregistration call, no bookkeeping.
+//! ## Joining and reading a group
 //!
 //! ```
 //! use smarm::{channel, join, leave, members, pick, run, spawn};
@@ -25,8 +27,8 @@
 //!     join("pool", w2.pid());
 //!     assert_eq!(members("pool").len(), 2);
 //!
-//!     // One worker dies. Nobody told the group — the death hook evicts it,
-//!     // so it is gone from `members` and never returned by `pick`.
+//!     // One worker dies. Nothing tells the group — smarm evicts it
+//!     // automatically, so it is gone from `members` and never picked.
 //!     tx1.send(()).unwrap();
 //!     w1.join().unwrap();
 //!     assert_eq!(members("pool"), vec![w2.pid()]);
@@ -41,60 +43,72 @@
 //! });
 //! ```
 //!
-//! ## Cleanup is eager and monitor-driven (unlike the registry)
+//! [`members`] returns every live member in the order they joined; [`pick`]
+//! returns one of them, or `None` when the group is empty. The same actor can
+//! belong to any number of groups at once, and joining a group it is already in
+//! is a harmless no-op.
 //!
-//! The registry prunes a stale binding lazily, on contact, because it only
-//! ever resolves one binding at a time. A group is *iterated* — `members` fans
-//! out to every member — so it must not carry dead members across a broadcast.
-//! Each [`join`] installs a `monitor(pid)` and keeps the resulting [`Monitor`]
-//! *alongside the group entry*; the actor's one-shot `Down` lands in that
-//! monitor's channel when it dies. Every group operation [`reap`s][reap] the
-//! group it touches first — draining each membership's monitor with a
-//! non-blocking `try_recv` — and on the first sign of death sweeps that pid out
-//! of *every* group via the predicate primitive. So the set is self-pruning on
-//! contact rather than filtered on read; the read path additionally applies a
-//! generation-checked liveness backstop so a member already dead but not yet
-//! reaped is never *returned*, even though eviction remains the monitor's job.
+//! ## Membership ends on its own
 //!
-//! [reap]: ProcessGroups::reap_group
+//! You do not have to clean up after a member that dies. When an actor exits —
+//! for any reason — it is removed from every group it had joined, before any
+//! later read or send can observe it. [`leave`] is only for *voluntary*
+//! departure, when a still-living actor wants out of a group.
 //!
-//! ## Eviction is one dumb primitive
+//! This is the main difference from keeping your own `Vec<Pid>`: a plain list
+//! goes stale the instant a member dies, and you would have to notice and prune
+//! it yourself. A group prunes itself.
 //!
-//! [`ProcessGroups::remove_where`] removes every member matching a predicate
-//! from every group. The primitive never knows *why* a member leaves — that is
-//! the caller's concern. Its first caller is the monitor death hook (this RFC,
-//! via `reap_group`); the later `evict_incarnation(node, inc)` sweep (RFC 010)
-//! reuses the same predicate path, which is the whole reason to shape it as a
-//! predicate.
+//! ## Sending to a group
 //!
-//! ## Identity is cluster-shaped from the first commit
+//! For a worker pool you usually want to hand a job to *one* available member.
+//! [`dispatch`] picks a live member and sends it a message in a single step,
+//! returning the member it reached:
 //!
-//! A [`Member`] is not a bare [`Pid`]: it is `(NodeId, Incarnation, Pid)`, a
-//! field-for-field image of a modern BEAM pid (`NEW_PID_EXT`) so the eventual
-//! ETF codec (RFC 011) is a near-identity mapping. In this RFC `NodeId` and
-//! `Incarnation` are runtime-init constants — one node, one fixed incarnation
-//! — threaded through storage and eviction anyway so the public surface never
-//! has to change to acquire them once clustering (RFC 010) supplies real
-//! values. No cluster types leak out of the public functions: callers pass and
-//! receive [`Pid`]; the node/incarnation are filled from runtime identity.
+//! ```ignore
+//! use smarm::{dispatch, join, Addressable};
 //!
-//! ## Locking
+//! struct Job(String);
+//! struct Worker;
+//! impl Addressable for Worker { type Msg = Job; }
 //!
-//! One `RawMutex` (Leaf class) on `RuntimeInner`, mirroring `registry`. The
-//! group lock is never held with another Leaf (it never touches the registry
-//! or a slot's cold lock). The two places that *do* need another lock are kept
-//! off the group-lock path:
+//! // Each worker has published a `Pid<Worker>` inbox and joined the pool.
+//! join("workers", worker_a);
+//! join("workers", worker_b);
 //!
-//!   - `monitor()` / `demonitor()` acquire the target's cold lock (Leaf) and
-//!     so run *before* / *after* the group lock, never under it.
-//!   - draining a monitor with `try_recv` takes the channel's Channel-class
-//!     lock — permitted *under* a Leaf by the lock order (`raw_mutex.rs`), and
-//!     a channel critical section only does the lock-free unpark protocol, so
-//!     no Leaf is ever nested under it.
+//! // Route one job to whichever live worker `pick` lands on.
+//! match dispatch::<Worker>("workers", Job("resize image".into())) {
+//!     Ok(who) => println!("sent to {who:?}"),
+//!     Err(returned) => println!("no worker took it: {returned:?}"),
+//! }
+//! ```
 //!
-//! Evicted and rejected [`Monitor`]s are dropped only *after* the group lock is
-//! released, so a receiver-drop never runs a wakeup under the lock (same
-//! discipline as `demonitor`).
+//! When you want the pids themselves rather than to send right away, [`pick_as`]
+//! and [`members_as`] return typed [`Pid<A>`](Pid)s for a homogeneous group, so
+//! the follow-up send stays compile-checked. The untyped [`pick`] and
+//! [`members`] are for mixed groups, where all you can rely on is identity.
+//!
+//! ## Groups vs. the registry
+//!
+//! A group is the many-actors counterpart to the [`registry`](crate::registry).
+//! The registry binds a name to *at most one* actor and re-resolves it on every
+//! send — what you want for a single well-known service. A group binds a name to
+//! *many* actors, and one actor may sit in many groups. Reach for the registry
+//! when there is exactly one of something; reach for a group when there is a set.
+//!
+//! ## Identity and clustering
+//!
+//! A group member is described by a [`Member`] — a [`Pid`] plus a [`NodeId`] and
+//! an [`Incarnation`]. Today everything is single-node, those two fields are
+//! fixed defaults, and you only ever pass and receive a plain [`Pid`]: the extra
+//! identity is carried so this API will not have to change when groups learn to
+//! span a cluster.
+//!
+//! ## Running context
+//!
+//! Every function here addresses the current runtime, so each must be called
+//! from inside [`run`](crate::run) (that is, on an actor thread). Calling one
+//! from outside a running runtime panics.
 
 use crate::monitor::{demonitor, monitor, Monitor};
 use crate::pid::{assert_type, Addressable, Pid};
@@ -103,7 +117,7 @@ use crate::scheduler::with_runtime;
 use std::collections::HashMap;
 
 /// A cluster node handle. A `u32` integer handle, *not* an interned atom — the
-/// single deliberate divergence from the BEAM wire shape (RFC 011 names it).
+/// single deliberate divergence from the BEAM wire shape.
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
 pub struct NodeId(u32);
 
@@ -149,9 +163,8 @@ impl From<u32> for Incarnation {
     }
 }
 
-/// The fixed single-node identity used until clustering (RFC 010) supplies
-/// real values. Carried like `wake_slot` so the public API never has to change
-/// to acquire it.
+/// The fixed single-node identity used until clustering supplies real values.
+/// Carried like `wake_slot` so the public API never has to change to acquire it.
 pub const DEFAULT_NODE_ID: NodeId = NodeId(0);
 /// The fixed incarnation for the single-node default. Non-zero so it never
 /// collides with a BEAM "any creation" wildcard at interop time.
@@ -161,7 +174,7 @@ pub const DEFAULT_INCARNATION: Incarnation = Incarnation(1);
 ///
 /// Deliberately a field-for-field image of a modern BEAM pid (`NEW_PID_EXT`).
 /// In memory it is a plain struct — no wire packing; the packed representation
-/// belongs to the `RemoteRef` boundary (RFC 010/011), not here.
+/// belongs to the remote-reference boundary, not here.
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
 pub struct Member {
     /// Which node the pid lives on. `DEFAULT_NODE_ID` while single-node.
@@ -174,7 +187,7 @@ pub struct Member {
 }
 
 /// One membership: a [`Member`] and the [`Monitor`] that watches its liveness.
-/// The monitor lives *alongside* the group entry (RFC 012) so a group is
+/// The monitor lives *alongside* the group entry so a group is
 /// self-contained: draining the membership tells us whether the member is
 /// still alive, and dropping the membership drops its monitor.
 struct Membership {
@@ -185,7 +198,23 @@ struct Membership {
 /// The store: `name → multiset<Member>`. Within a single group a `Member`
 /// appears at most once (`join` is idempotent); the *multiset* framing is for
 /// cluster-readiness — the same pid is freely a member of many groups, and the
-/// width admits multiples in general. Held under one Leaf-class `RawMutex`.
+/// width admits multiples in general.
+///
+/// Locking discipline. Held under one Leaf-class `RawMutex` on `RuntimeInner`,
+/// mirroring the registry, and never held together with another Leaf lock (it
+/// never touches the registry or a slot's cold lock). The two operations that
+/// do need another lock are kept off the group-lock path:
+///
+///   - `monitor()` / `demonitor()` take the target's cold lock (also Leaf), so
+///     they run *before* / *after* the group lock, never under it.
+///   - draining a monitor with `try_recv` takes the channel's Channel-class
+///     lock, which the lock order permits *under* a Leaf; a channel critical
+///     section only does the lock-free unpark protocol, so no Leaf ever nests
+///     under it.
+///
+/// Evicted and rejected [`Monitor`]s are therefore dropped only *after* the
+/// group lock is released, so a receiver-drop never runs a wakeup under the
+/// lock — the same discipline as `demonitor`.
 pub(crate) struct ProcessGroups {
     groups: HashMap<String, Vec<Membership>>,
 }
@@ -223,9 +252,11 @@ impl ProcessGroups {
     /// The one dumb eviction primitive: drop every member matching `pred` from
     /// every group, pruning emptied groups, and return the evicted memberships'
     /// monitors for the caller to drop outside the lock. The primitive does not
-    /// know *why* a member leaves. Callers: the death hook (`reap_group`) and,
-    /// later, `evict_incarnation` (RFC 010), over the same path. Insertion
-    /// order within a group is preserved (`members` / `pick` are order-stable).
+    /// know *why* a member leaves; that is the caller's concern. Its callers are
+    /// the death hook (`reap_group`) and, once clustering lands, an
+    /// incarnation-eviction sweep — both over this same predicate path, which is
+    /// the whole reason to shape eviction as a predicate. Insertion order within
+    /// a group is preserved (`members` / `pick` are order-stable).
     fn remove_where(&mut self, mut pred: impl FnMut(&Member) -> bool) -> Vec<Monitor> {
         let mut evicted = Vec::new();
         self.groups.retain(|_, v| {
@@ -242,12 +273,18 @@ impl ProcessGroups {
         evicted
     }
 
-    /// Drain-on-contact death hook. Drains every membership monitor in `group`
-    /// with a non-blocking `try_recv`: a delivered `Down` (any reason) or a
-    /// closed channel means that member is dead. On the first death detected,
-    /// sweep *all* of the dead pids out of *every* group via [`remove_where`]
-    /// — a death is removed from each group it joined, not just the one being
-    /// touched. Returns the evicted monitors to drop outside the lock.
+    /// Drain-on-contact death hook. The registry can prune a stale binding
+    /// lazily, on contact, because it only ever resolves one binding at a time;
+    /// a group is *iterated* — `members` fans out to everyone — so it must not
+    /// carry a dead member across a broadcast. Every group operation reaps the
+    /// group it touches first.
+    ///
+    /// Drains every membership monitor in `group` with a non-blocking
+    /// `try_recv`: a delivered `Down` (any reason) or a closed channel means
+    /// that member is dead. On the first death detected, sweep *all* of the
+    /// dead pids out of *every* group via [`remove_where`] — a death is removed
+    /// from each group it joined, not just the one being touched. Returns the
+    /// evicted monitors to drop outside the lock.
     fn reap_group(&mut self, group: &str) -> Vec<Monitor> {
         let dead: Vec<Pid> = {
             let Some(v) = self.groups.get(group) else {
@@ -279,7 +316,7 @@ impl ProcessGroups {
     }
 
     /// Live members of `group`, in insertion order. The `is_live` oracle is the
-    /// read-path backstop (Phase 3): a member whose slot is already dead is
+    /// read-path backstop: a member whose slot is already dead is
     /// dropped from the *result* even if its `Down` has not been drained yet.
     /// Backstop only — the entry stays in storage; eviction is the monitor's
     /// job (`reap_group`).
@@ -314,18 +351,19 @@ fn live(inner: &crate::runtime::RuntimeInner, pid: Pid) -> bool {
 /// pid is a member at most once (idempotent). Returns `true` if this call newly
 /// added the membership, `false` if it was already a member.
 ///
-/// Installs a `monitor(pid)` whose one-shot `Down` drives eviction: the
-/// registration races `finalize_actor` under the slot's cold lock exactly as
-/// every other monitor does, so no death slips between the join and the
-/// registration. A redundant (idempotent) join tears its extra monitor back
-/// down.
+/// Installs a monitor on `pid` so the actor's death evicts it from the group
+/// automatically — you never have to remove a dead member yourself. A redundant
+/// (idempotent) join tears its extra monitor back down.
 ///
 /// Panics if called outside `Runtime::run()`.
 pub fn join<A>(group: impl Into<String>, pid: Pid<A>) -> bool {
     let group = group.into();
     let pid = pid.erase();
     // Install the monitor BEFORE taking the group lock: monitor() acquires the
-    // target's cold lock (Leaf), and two Leaf locks are never held at once.
+    // target's cold lock (Leaf), and two Leaf locks are never held at once. The
+    // registration races `finalize_actor` under that cold lock exactly as every
+    // other monitor does, so no death can slip between the join and the monitor
+    // being in place.
     let mon = monitor(pid);
 
     let (rejected, reaped) = with_runtime(|inner| {
@@ -372,12 +410,13 @@ pub fn leave<A>(group: &str, pid: Pid<A>) -> bool {
     }
 }
 
-/// Fan-out read: every live member of `group`.
+/// Every live member of `group`, in the order they joined. Returns an empty
+/// vector if the group does not exist or has no live members.
 ///
-/// The touched group is reaped first, so dead members are evicted before the
-/// read. The generation-checked liveness read is a belt-and-braces backstop:
-/// even in the finalize window where a member is already dead but its `Down`
-/// has not yet landed, it is dropped from the result.
+/// Dead members are never returned: the group is pruned of anything that has
+/// died before the read, and as a backstop a member whose slot is already dead
+/// is dropped from the result even in the brief window before its death has
+/// been fully processed.
 ///
 /// Panics if called outside `Runtime::run()`.
 pub fn members(group: &str) -> Vec<Pid> {
@@ -391,11 +430,10 @@ pub fn members(group: &str) -> Vec<Pid> {
     pids
 }
 
-/// Pool/discovery read: one live member of `group`, or `None` if empty.
-/// Stateless first-live selection: the touched group is reaped first, and the
-/// generation-checked liveness backstop skips any member already dead but not
-/// yet reaped. Smarter routing is an explicitly later, clustered concern per
-/// RFC 010.
+/// One live member of `group`, or `None` if the group is empty (or every
+/// member has died). Selection is a stateless first-live scan in join order,
+/// with the same dead-member backstop as [`members`]; smarter, load-aware
+/// routing is a later, clustered concern.
 ///
 /// Panics if called outside `Runtime::run()`.
 pub fn pick(group: &str) -> Option<Pid> {
@@ -409,11 +447,11 @@ pub fn pick(group: &str) -> Option<Pid> {
     picked
 }
 
-/// Typed `pick`: one live member of `group` as a [`Pid<A>`] (RFC 014 §4.4).
+/// Typed [`pick`]: one live member of `group` as a [`Pid<A>`](Pid).
 /// For a homogeneous pool every member is an `A`, so the picked member comes
 /// back typed and dispatch is an ordinary compile-checked [`send_to`] rather
 /// than the [`send_dyn`](crate::send_dyn) escape hatch. Re-types via the
-/// unchecked [`assert_type`] primitive — a wrong `A` degrades to
+/// unchecked `assert_type` primitive — a wrong `A` degrades to
 /// [`SendError::NoChannel`] on the next send, never a misdelivery.
 ///
 /// Panics if called outside `Runtime::run()`.

src/preempt.rs

@@ -98,6 +98,25 @@ pub(crate) fn clear_current_slot() {
     CURRENT_SLOT.with(|c| c.set(std::ptr::null()));
 }
 
+/// RFC 007 (`smarm-causal`) — raw pointer to the on-CPU actor's slot, null on
+/// the scheduler's own stack. Same lifetime argument as `note_overrun`: the
+/// slot is never reclaimed while its actor is on-CPU.
+#[cfg(feature = "smarm-causal")]
+#[inline]
+pub(crate) fn current_slot_ptr() -> *const crate::runtime::Slot {
+    CURRENT_SLOT.with(|c| c.get())
+}
+
+/// RFC 007 (`smarm-causal`) — push the slice start forward by `cycles`, so
+/// virtually-injected delay spun inside `maybe_preempt` does not count against
+/// the actor's timeslice (the clock-correction half of the RFC: the runtime
+/// owns this clock, so it can subtract its own perturbation).
+#[cfg(feature = "smarm-causal")]
+#[inline]
+pub(crate) fn extend_timeslice(cycles: u64) {
+    TIMESLICE_START.with(|c| c.set(c.get().wrapping_add(cycles)));
+}
+
 /// Tally a timeslice overrun against the on-CPU actor (RFC 016 Chunk 2). A
 /// no-op if no actor is bound (the scheduler's own stack). Reached only from
 /// the slice-expiry branch, which is already the yield path, so its cost is
@@ -247,6 +266,11 @@ pub fn maybe_preempt() {
                 // Observe a pending stop first: if we are being cancelled
                 // there is no point yielding, we unwind instead.
                 check_cancelled();
+                // RFC 007: causal-profiling sample/absorb point. Shares the
+                // amortised cadence, and the PREEMPTION_ENABLED gate — so it
+                // can never spin inside a prep-to-park or no-preempt region.
+                #[cfg(feature = "smarm-causal")]
+                crate::causal::check();
                 let start = TIMESLICE_START.with(|s| s.get());
                 if rdtsc().saturating_sub(start) > CONFIGURED_TIMESLICE_CYCLES.with(|t| t.get()) {
                     // Tally the overrun (RFC 016 Chunk 2) before handing back —

src/registry.rs

@@ -1,55 +1,108 @@
-//! Named mailbox registry — resolve a name (or pid) to a *messageable* actor.
+
+//! Give an actor a name so other actors can find it and message it.
 //!
-//! ## What changed (RFC 014)
+//! Without the registry, the only way to reach an actor is to already be
+//! holding its [`Pid`], usually because you spawned it yourself or someone
+//! passed it to you. That is fine for a worker you just created, but it does
+//! not work for a well-known service that arbitrary parts of your program
+//! need to find independently, like a logger, a config store, or a
+//! connection pool. The registry solves this: an actor claims a name once,
+//! and from then on any other actor can look that name up, or send to it
+//! directly, without ever having been handed a `Pid`.
 //!
-//! The old registry was a `name <-> pid` bimap: `whereis` handed back a `Pid`
-//! you could not send to, because a pid is just `(index, generation)` with no
-//! delivery endpoint. This rework makes resolution yield something messageable.
+//! ```
+//! use smarm::{channel, register, run, send, spawn, unregister, whereis, Name};
 //!
-//! Two facts shape the structure:
+//! const COUNTER: Name<u64> = Name::new("counter");
 //!
-//! 1. **A name resolves to a single actor.** Many actors under one label is
-//!    what *process groups* (`pg`) are for; the registry is one-name-one-actor
-//!    (several names *may* point at the same actor).
-//! 2. **Channels are typed**, so an actor has no single untyped mailbox. An
-//!    actor instead owns a *set* of typed channels — one [`Sender`] per message
-//!    type it accepts. So the registry maps name/pid to a [`Mailbox`]: a small
-//!    structure holding that actor's pid plus all of its typed channels, keyed
-//!    by message [`TypeId`].
+//! run(|| {
+//!     let (ready_tx, ready_rx) = channel::<()>();
+//!     let (tx, rx) = channel::<u64>();
 //!
-//! Resolution is therefore: `name -> pid` (single actor) `-> Mailbox -> the
-//! channel for message type M`. A `Name<Cmd>` and a `Name<Admin>` on the *same*
-//! actor select *different* channels purely by their type parameter, so
-//! capability separation (RFC 014 §4.7) needs no extra machinery.
+//!     let worker = spawn(move || {
+//!         // Claim the name for this actor's inbox. Any actor holding
+//!         // `COUNTER` can now reach this one by name.
+//!         register(COUNTER, tx).unwrap();
+//!         ready_tx.send(()).unwrap();
+//!         assert_eq!(rx.recv().unwrap(), 42);
+//!     });
 //!
-//! ## Type erasure is contained
+//!     ready_rx.recv().unwrap(); // wait for the worker to register
 //!
-//! Each stored channel is a `Box<dyn Any + Send>` that is concretely a
-//! `Sender<M>`, filed under `TypeId::of::<M>()`. A resolve for `M` looks up
-//! that exact `TypeId` and downcasts to `Sender<M>` — keyed by the very type we
-//! downcast to, so the downcast cannot fail on correct data; a failure is a
-//! smarm bug, asserted in debug. The phantom `M` on [`Name`] re-imposes the
-//! type at the call site, so callers never touch the erasure.
+//!     // Look the name up, or just send to it directly.
+//!     assert_eq!(whereis("counter"), Some(worker.pid()));
+//!     send(COUNTER, 42).unwrap();
 //!
-//! ## Cleanup is lazy (prune-on-contact)
+//!     worker.join().unwrap();
 //!
-//! As before, there is no `finalize` hook and no name field on the slot. Every
-//! operation that touches a binding checks the target pid's liveness via the
-//! generation-checked slot word; a binding to a dead actor behaves as absent
-//! and is pruned on contact (its [`Mailbox`] and every name pointing at it are
-//! dropped). The cost is a dead binding lingering until something looks at it;
-//! the payoff is zero coupling to the actor lifecycle.
+//!     // The name dies with the actor: nobody holds it anymore.
+//!     assert_eq!(whereis("counter"), None);
+//! });
+//! ```
 //!
-//! ## Locking
+//! ## Names carry a message type
 //!
-//! One `RawMutex` (Leaf class) in `RuntimeInner`, exactly like the old
-//! registry. The fold (name index *and* handles under the one lock) is what
-//! keeps a name-addressed `send` on a single Leaf — `raw_mutex` panics on a
-//! second Leaf acquired while one is held. The send path clones the `Sender`
-//! **under** the Leaf lock (a `Sender::clone` takes a Channel lock, permitted
-//! under a Leaf), then **releases** the Leaf and only *then* sends — a send can
-//! unpark a receiver, and wakeup-bearing work runs outside the Leaf. Order is
-//! **Leaf -> Channel**, as `pg`/`finalize`.
+//! A [`Name<M>`] is a plain string plus a type parameter `M`: the message
+//! type that name expects to receive. [`Name::new`] is `const`, so the usual
+//! pattern is a module-level constant like `COUNTER` above, shared by every
+//! caller. The type parameter means a name is only ever sent the kind of
+//! message it was declared for. If two different constants share the same
+//! string but have different message types, they still address two
+//! independent channels on the same actor: registering both just gives that
+//! actor two ways to be reached, one per message type. This is how you give
+//! one actor a "public" channel and a separate, differently-typed "admin"
+//! channel under related names, without inventing an enum to merge them.
+//!
+//! ## One actor per name, looked up fresh every time
+//!
+//! A name always points at exactly one actor at a time (contrast a *process
+//! group*, from the [`pg`](crate::pg) module, which is one name mapping to
+//! many actors). Unlike a plain [`Pid`], which names one specific actor
+//! forever and stops working the moment that actor dies, a name is
+//! re-resolved on every [`send`]: if the actor holding it dies and a new one
+//! registers under the same name, the next `send` reaches the new holder
+//! automatically. Use a name for a long-lived service whose exact identity
+//! you do not want to track by hand; use a `Pid` when you already have one
+//! and want to talk to that exact actor.
+//!
+//! ## Registration ends when the actor does
+//!
+//! There is no separate step to clean up a name when its actor exits: dying
+//! is enough. The next operation that touches a dead binding (a [`whereis`],
+//! a [`send`], or another actor's [`register`] of the same name) notices the
+//! actor is gone and clears the stale entry as a side effect, so the name
+//! becomes free again. [`unregister`] is only for a live actor voluntarily
+//! giving up a name it no longer wants; nothing has to call it on the way
+//! out.
+//!
+//! ## Implementation notes
+//!
+//! These details matter if you are working on smarm itself; they are not
+//! part of the public contract.
+//!
+//! Internally, each live actor that has published at least one channel owns
+//! a `Mailbox`: its pid plus a set of typed channels, keyed by the message
+//! type's `TypeId`. A stored channel is a `Box<dyn Any + Send>` that
+//! is concretely a `Sender<M>`; resolving for `M` looks up that exact
+//! `TypeId` and downcasts, so the downcast cannot fail on correct data (a
+//! failure would be a bug in the registry itself, checked in debug builds).
+//! Registering a name therefore means: find or create the actor's mailbox,
+//! insert the channel under its type, and point the name at the actor's pid.
+//!
+//! There is no callback when an actor exits. Every operation that touches a
+//! binding checks the target pid's liveness directly against the scheduler's
+//! slot table (which also tracks a generation counter, so a dead actor's
+//! reused slot index is never mistaken for the same actor). A binding to a
+//! dead actor is treated as absent and dropped right there. This keeps the
+//! registry decoupled from actor teardown, at the cost of a dead binding
+//! lingering until something happens to look at it.
+//!
+//! The whole registry (both the name index and the per-actor mailboxes) sits
+//! behind one lock, which is what lets a name-addressed [`send`] resolve and
+//! clone the target's sender in a single critical section. The sender is
+//! cloned while that lock is held, then the lock is released before the
+//! actual send, since delivering a message can wake a parked receiver and
+//! that wakeup work should not run while the registry is locked.
 
 use crate::channel::Sender;
 use crate::pid::{Addressable, Name, Pid};
@@ -80,28 +133,33 @@ impl std::fmt::Display for RegisterError {
 
 impl std::error::Error for RegisterError {}
 
-/// Why a name-addressed [`send`] did not deliver. Carries the message back so
-/// the caller never loses it (mirrors [`crate::channel::SendError`]).
+/// Why a send did not deliver. Every variant carries the undelivered message
+/// back, mirroring [`crate::channel::SendError`], so a failed send never
+/// silently drops what you tried to send.
 ///
-/// `Debug`/`Display` are hand-written so neither demands `M: Debug` — the
-/// payload is returned, not printed.
+/// `Debug` and `Display` are hand-written so neither requires `M: Debug`,
+/// since the payload is handed back to you, not printed.
 pub enum SendError<M> {
-    /// No live actor is currently registered under this name. Name-addressed
-    /// [`send`] only; the pid-addressed counterpart is [`SendError::Dead`].
+    /// No live actor is currently registered under this name. Returned only
+    /// by name-addressed [`send`]; the pid-addressed counterpart of "nothing
+    /// there" is [`SendError::Dead`].
     Unresolved(M),
-    /// The pid-addressed actor is no longer the live incarnation this pid names
-    /// — it has died, even if its slot now holds a *different* actor (a direct
-    /// `Pid<A>` send never redirects; contrast name-addressed [`send`]). Pid
-    /// paths ([`send_to`] / [`send_dyn`]) only.
+    /// The actor this pid identifies has died, even if its slot has since
+    /// been taken over by a different, live actor. A direct `Pid<A>` send
+    /// never redirects to that new occupant; contrast name-addressed
+    /// [`send`], which would reach it. Returned by the pid-addressed sends,
+    /// [`send_to`] and [`send_dyn`].
     Dead(M),
-    /// The actor is live but exposes no channel for this message type.
+    /// The actor is live but has not published a channel for this message
+    /// type.
     NoChannel(M),
-    /// The actor's channel for this message type is closed (its receiver is gone).
+    /// The actor's channel for this message type is closed (its receiver has
+    /// been dropped).
     Closed(M),
-    /// No live member to deliver to — a [`dispatch`](crate::dispatch) over an
-    /// empty (or all-dead) process group. Group-addressed dispatch only; the
-    /// name-addressed counterpart is [`SendError::Unresolved`]. The message is
-    /// handed back undelivered.
+    /// No live member was available to deliver to: returned by
+    /// [`dispatch`](crate::dispatch) when the target process group is empty
+    /// or every member in it has died. The name-addressed counterpart of
+    /// this case is [`SendError::Unresolved`].
     NoMember(M),
 }
 
@@ -150,9 +208,9 @@ impl<M> std::error::Error for SendError<M> {}
 
 /// A registry-stored channel, type-erased over its message type. The stored
 /// object must serve two readers: `clone_sender` (downcast back to the concrete
-/// `Sender<M>`) and the RFC 016 snapshot (queued length without knowing `M`).
-/// A bare `Box<dyn Any>` gives the first but not the second, so we erase behind
-/// this small trait instead.
+/// `Sender<M>`) and the runtime introspection snapshot (queued length without
+/// knowing `M`). A bare `Box<dyn Any>` gives the first but not the second, so
+/// we erase behind this small trait instead.
 trait ErasedSender: Send {
     fn as_any(&self) -> &dyn Any;
     fn queued_len(&self) -> usize;
@@ -169,7 +227,7 @@ impl<M: Send + 'static> ErasedSender for Sender<M> {
 
 /// One typed channel of an actor, type-erased. Concretely a `Sender<M>` filed
 /// under `TypeId::of::<M>()`; `msg_type` is `type_name::<M>()`, kept for
-/// observers (RFC 014 §4.5) and as the debug cross-check on the downcast.
+/// observability tooling and as the debug cross-check on the downcast.
 struct Channel {
     sender: Box<dyn ErasedSender>,
     msg_type: &'static str,
@@ -193,17 +251,18 @@ impl Mailbox {
     /// legal under a Leaf (Leaf -> Channel).
     fn clone_sender<M: Send + 'static>(&self) -> Option<Sender<M>> {
         let ch = self.channels.get(&TypeId::of::<M>())?;
-        let tx = ch
-            .sender
-            .as_any()
-            .downcast_ref::<Sender<M>>()
-            .expect("channel keyed by TypeId but downcast to its own type failed — smarm bug");
+        let tx = match ch.sender.as_any().downcast_ref::<Sender<M>>() {
+            Some(tx) => tx,
+            None => panic!(
+                "smarm: channel keyed by TypeId but downcast to its own type failed (core corrupt)"
+            ),
+        };
         debug_assert_eq!(ch.msg_type, type_name::<M>(), "msg_type / TypeId disagree");
         Some(tx.clone())
     }
 }
 
-/// Per-actor registry view handed to RFC 016 introspection: registered names
+/// Per-actor registry view handed to runtime introspection: registered names
 /// and summed mailbox depth, tagged with the mailbox's `pid` so a stale
 /// incarnation can be filtered against the slab. Covers only *published*
 /// channels (`register` / `install` / `spawn_addr` / gen_server start); an
@@ -216,14 +275,21 @@ pub(crate) struct MailboxInfo {
 }
 
 /// The directory. Invariant (held under the registry lock): every value in
-/// `by_name` is the index of a [`Mailbox`] present in `by_index`, and that
-/// mailbox's `pid.index()` equals the key. Stale entries (dead actors) violate
-/// nothing — they are simply pruned on contact.
+/// `by_name` is the full [`Pid`] (index *and* generation) of an actor that
+/// published a [`Mailbox`] into `by_index` at registration time. Stale entries
+/// (dead holders, including holders whose slot has since been re-tenanted by
+/// a different actor) violate nothing: they are pruned on contact, and the
+/// generation makes "dead" decidable even after slot reuse.
 pub(crate) struct Registry {
     /// `pid.index() -> the actor's mailbox`. The handle store.
     by_index: HashMap<u32, Mailbox>,
-    /// `name -> pid.index()`. Several names may map to one actor.
-    by_name: HashMap<&'static str, u32>,
+    /// `name -> holder pid`. Several names may map to one actor. The full pid
+    /// (not just the index) is load-bearing: an index alone cannot tell a dead
+    /// holder from the live actor now tenanting its recycled slot. Comparing
+    /// only the index would make such a name read as live-held (unresolvable
+    /// and unregisterable at once) and could misdeliver to whatever new,
+    /// same-typed actor now sits in that slot.
+    by_name: HashMap<&'static str, Pid>,
 }
 
 impl Registry {
@@ -231,24 +297,31 @@ impl Registry {
         Self { by_index: HashMap::new(), by_name: HashMap::new() }
     }
 
-    /// Drop a dead actor's mailbox and every name that pointed at it.
-    fn prune(&mut self, index: u32) {
-        self.by_index.remove(&index);
-        self.by_name.retain(|_, idx| *idx != index);
+    /// Drop a dead holder's artifacts: every name bound to it, and its
+    /// mailbox, but only while the mailbox is still *its own*. A recycled
+    /// slot's mailbox belongs to the live tenant (publish replaces it
+    /// wholesale on pid mismatch) and is left untouched.
+    fn prune_holder(&mut self, holder: Pid) {
+        self.by_name.retain(|_, p| *p != holder);
+        if self.by_index.get(&holder.index()).is_some_and(|mb| mb.pid == holder) {
+            self.by_index.remove(&holder.index());
+        }
     }
 
-    /// RFC 016 snapshot input: per-slot-index registry view — the actor's
-    /// registered names (inverted from `by_name`) and its mailbox depth (queued
-    /// messages summed across every published typed channel). Built in one pass
-    /// under the registry Leaf; the per-channel `queued_len` takes a Channel
-    /// lock, legal under the Leaf (Leaf → Channel). Carries each mailbox's full
-    /// `pid` so the caller can discard a stale incarnation's entry against the
-    /// slab's live generation. Names that dangle (point at no mailbox) are
-    /// dropped — they violate no invariant and get pruned on next contact.
+    /// Runtime introspection input: per-slot-index registry view, giving the
+    /// actor's registered names (inverted from `by_name`) and its mailbox
+    /// depth (queued messages summed across every published typed channel).
+    /// Carries each mailbox's full `pid` so the caller can discard a stale
+    /// incarnation's entry against the slab's live generation. Names are
+    /// matched to mailboxes by *full pid*, so a stale name (dead holder)
+    /// still annotates the corpse's own mailbox if that survives, but never a
+    /// recycled slot's new tenant; names that attach to no mailbox are
+    /// dropped, since that violates no invariant and they get pruned on next
+    /// contact.
     pub(crate) fn introspect_map(&self) -> HashMap<u32, MailboxInfo> {
-        let mut names: HashMap<u32, Vec<&'static str>> = HashMap::new();
-        for (&name, &idx) in &self.by_name {
-            names.entry(idx).or_default().push(name);
+        let mut names: HashMap<Pid, Vec<&'static str>> = HashMap::new();
+        for (&name, &pid) in &self.by_name {
+            names.entry(pid).or_default().push(name);
         }
         let mut out: HashMap<u32, MailboxInfo> = HashMap::with_capacity(self.by_index.len());
         for (&idx, mb) in &self.by_index {
@@ -257,7 +330,7 @@ impl Registry {
                 idx,
                 MailboxInfo {
                     pid: mb.pid,
-                    names: names.remove(&idx).unwrap_or_default(),
+                    names: names.remove(&mb.pid).unwrap_or_default(),
                     depth: depth.min(u32::MAX as usize) as u32,
                 },
             );
@@ -267,15 +340,16 @@ impl Registry {
 
     /// Single-actor form of [`introspect_map`](Self::introspect_map): the
     /// registry view for one slot index, or `None` if no mailbox is published
-    /// there. Used by `actor_info` so its cost stays proportional to the one
-    /// actor rather than locking every channel in the runtime.
+    /// there. Used by the runtime's per-actor introspection so its cost stays
+    /// proportional to the one actor rather than locking every channel in the
+    /// runtime.
     pub(crate) fn introspect_one(&self, idx: u32) -> Option<MailboxInfo> {
         let mb = self.by_index.get(&idx)?;
         let depth: usize = mb.channels.values().map(|c| c.sender.queued_len()).sum();
         let names = self
             .by_name
             .iter()
-            .filter_map(|(&n, &i)| (i == idx).then_some(n))
+            .filter_map(|(&n, &p)| (p == mb.pid).then_some(n))
             .collect();
         Some(MailboxInfo { pid: mb.pid, names, depth: depth.min(u32::MAX as usize) as u32 })
     }
@@ -286,14 +360,18 @@ fn live(inner: &crate::runtime::RuntimeInner, pid: Pid) -> bool {
     inner.slot_at(pid).is_some_and(|s| s.is_live_for(pid))
 }
 
-/// Publish the current actor's `Sender<M>` under `name`, capturing the channel
-/// so the name becomes messageable. Idempotent for the same `(name, type)`;
-/// registering a *second* type under the same (or another) name on the same
-/// actor just adds another channel to the actor's mailbox.
+/// Give the current actor's channel a name, so other actors can find and
+/// message it by that name instead of needing its [`Pid`].
 ///
-/// Fails with [`RegisterError::NameTaken`] if the name is held by a *different*
-/// live actor (a binding to a dead actor is pruned and the name treated as
-/// free). Panics if called outside `Runtime::run()`.
+/// Calling this again with the same `(name, type)` from the same actor is
+/// harmless. Registering a *second* message type under the same (or a
+/// different) name from the same actor just adds another typed channel to
+/// that actor's mailbox; it does not replace the first.
+///
+/// Fails with [`RegisterError::NameTaken`] if the name is currently held by a
+/// *different* live actor. A name held by an actor that has since died is not
+/// considered taken: it is quietly reclaimed and handed to you. Panics if
+/// called outside [`run`](crate::run).
 pub fn register<M: Send + 'static>(name: Name<M>, tx: Sender<M>) -> Result<(), RegisterError> {
     register_with(self_pid(), name.as_str(), tx)
 }
@@ -301,8 +379,8 @@ pub fn register<M: Send + 'static>(name: Name<M>, tx: Sender<M>) -> Result<(), R
 /// Bind `name` to `pid`'s mailbox and publish `tx` under `M`'s [`TypeId`], for
 /// an explicit (already-live) actor rather than `self`. The shared core of
 /// [`register`] (which passes `self_pid()`) and the parent-side server-name
-/// bind in `gen_server` (which names a freshly spawned server before its body
-/// has run, so the name resolves the instant `start()` returns). Same collision
+/// bind in `gen_server`, which names a freshly spawned server before its body
+/// has run, so the name resolves the instant `start()` returns. Same collision
 /// rules and lock discipline as `register`.
 pub(crate) fn register_with<M: Send + 'static>(
     me: Pid,
@@ -314,21 +392,22 @@ pub(crate) fn register_with<M: Send + 'static>(
         if !live(inner, me) {
             return Err(RegisterError::NoProc);
         }
-        if let Some(&holder_idx) = reg.by_name.get(key) {
-            match reg.by_index.get(&holder_idx).map(|m| m.pid) {
-                Some(holder) if holder == me => {} // same actor: just add the channel below
-                Some(holder) if live(inner, holder) => {
-                    return Err(RegisterError::NameTaken { holder });
-                }
-                Some(_) => reg.prune(holder_idx), // dead holder: free the name
-                None => {
-                    reg.by_name.remove(key); // dangling name: free it
-                }
+        if let Some(&holder) = reg.by_name.get(key) {
+            if holder == me {
+                // Same actor: just add the channel below.
+            } else if live(inner, holder) {
+                return Err(RegisterError::NameTaken { holder });
+            } else {
+                // Dead holder: free the name (and its other stale artifacts).
+                // Liveness is judged against the *stored* pid, generation
+                // included, so a recycled slot's live tenant no longer makes a
+                // dead name read as taken.
+                reg.prune_holder(holder);
             }
         }
         // Publish (or extend) the mailbox with this channel, then bind the name.
         publish_channel::<M>(&mut reg, me, tx);
-        reg.by_name.insert(key, me.index());
+        reg.by_name.insert(key, me);
         Ok(())
     })
 }
@@ -351,14 +430,14 @@ fn publish_channel<M: Send + 'static>(reg: &mut Registry, me: Pid, tx: Sender<M>
 
 /// Publish the current actor's `Sender<A::Msg>` into its mailbox **without**
 /// binding a name, and hand back the typed [`Pid<A>`] that addresses this
-/// actor directly. This is the opt-in, lazy install of RFC 014 §5: an actor
-/// that wants to be reachable by a direct, identity-bound [`Pid<A>`] (rather
-/// than only via a re-resolving [`Name`]) calls this once with its inbox
-/// sender, then hands the returned pid out.
+/// actor directly.
 ///
-/// Unlike [`register`] there is no name to collide on, and `self` is always a
-/// live actor inside `run()`, so this is infallible. Panics if called outside
-/// `Runtime::run()`.
+/// This is for an actor that wants to be reachable directly by its pid,
+/// rather than only through a re-resolving [`Name`]: call this once with your
+/// inbox sender, then hand the returned `Pid<A>` to whoever should be able to
+/// message you. Unlike [`register`] there is no name to collide on, and the
+/// current actor is always live while inside `run()`, so this cannot fail.
+/// Panics if called outside [`run`](crate::run).
 pub fn install<A: Addressable>(tx: Sender<A::Msg>) -> Pid<A> {
     let me = self_pid();
     with_runtime(|inner| {
@@ -374,11 +453,12 @@ pub fn install<A: Addressable>(tx: Sender<A::Msg>) -> Pid<A> {
 /// Publish `tx` into `pid`'s mailbox under `M`'s [`TypeId`], for an explicit
 /// (freshly minted, already-live) actor rather than `self`. The parent-side
 /// half of [`spawn_addr`](crate::spawn_addr): the spawner makes the inbox and
-/// publishes the sender here *before* handing back the `Pid<A>`, so an immediate
-/// `send_to` on the returned pid always resolves — the address is live the
-/// instant the caller holds it, with no dependence on the body having run yet.
+/// publishes the sender here *before* handing back the `Pid<A>`, so an
+/// immediate `send_to` on the returned pid always resolves. The address is
+/// live the instant the caller holds it, with no dependence on the spawned
+/// actor's body having run yet.
 ///
-/// Caller guarantees `pid` is the just-installed actor (Queued, this exact
+/// Caller guarantees `pid` is the just-installed actor (queued, this exact
 /// incarnation); `publish_channel` replaces any stale leftover at the slot.
 pub(crate) fn install_for<M: Send + 'static>(pid: Pid, tx: Sender<M>) {
     with_runtime(|inner| {
@@ -388,109 +468,106 @@ pub(crate) fn install_for<M: Send + 'static>(pid: Pid, tx: Sender<M>) {
     });
 }
 
-/// The single actor currently registered under `name`, or `None` if unbound or
-/// no longer live (the stale binding is pruned on the way out).
+/// Look up which actor currently holds `name`, if any. Returns `None` if the
+/// name is unbound, or if it was bound to an actor that has since died (the
+/// stale binding is cleared as a side effect of this call).
 pub fn whereis(name: &str) -> Option<Pid> {
     with_runtime(|inner| {
         let mut reg = inner.registry.lock();
-        let idx = *reg.by_name.get(name)?;
-        match reg.by_index.get(&idx).map(|m| m.pid) {
-            Some(pid) if live(inner, pid) => Some(pid),
-            Some(_) => {
-                reg.prune(idx);
-                None
-            }
-            None => {
-                reg.by_name.remove(name);
-                None
-            }
+        let pid = *reg.by_name.get(name)?;
+        if live(inner, pid) {
+            Some(pid)
+        } else {
+            // Generation-checked against the stored holder: a recycled slot's
+            // live tenant reads dead here, and the stale name heals.
+            reg.prune_holder(pid);
+            None
         }
     })
 }
 
-/// Resolve `name` to a *typed* [`Pid<A>`] — the identity-bound counterpart of
-/// [`whereis`] (RFC 014 §4.4). Recovers the compile-checked
-/// [`send_to`] path from a durable name: looks the name up,
-/// then re-types the erased pid as `Pid<A>` via the unchecked
-/// [`assert_type`](crate::pid::assert_type) primitive. A wrong `A` is not
-/// unsound — it degrades to [`SendError::NoChannel`] on the next send (routing
-/// is by message `TypeId`), never a misdelivery. `None` if unbound or dead.
+/// Like [`whereis`], but returns a *typed* [`Pid<A>`] instead of a bare
+/// [`Pid`], so a follow-up [`send_to`] is compile-checked instead of needing
+/// the untyped [`send_dyn`] escape hatch. `None` if the name is unbound or its
+/// holder has died.
 ///
-/// Panics if called outside `Runtime::run()`.
+/// The type `A` is not checked against what the name's holder actually
+/// published: if you pick the wrong `A`, this still succeeds, but the next
+/// send against the returned pid degrades to [`SendError::NoChannel`] rather
+/// than reaching the wrong actor or the wrong channel.
+///
+/// Panics if called outside [`run`](crate::run).
 pub fn lookup_as<A: Addressable>(name: &str) -> Option<Pid<A>> {
     whereis(name).map(crate::pid::assert_type::<A>)
 }
 
-/// Resolve `name` to its actor's pid and a cloned `Sender<M>`, under the Leaf
-/// lock (clone-under-lock, then release). The crate-internal building block for
-/// `gen_server`'s by-name addressing: a named server publishes its inbox as a
-/// `Sender<Envelope<G>>` (via [`register_with`]), and `whereis_server` / `call`
-/// / `cast` recover that exact typed sender here to rebuild a `GenServerRef<G>`.
-/// `None` if unbound, dead (pruned on the way out), or holding no `M` channel.
+/// Resolve `name` to its actor's pid and a cloned `Sender<M>`, all under one
+/// lock acquisition. The crate-internal building block for `gen_server`'s
+/// by-name addressing: a named server publishes its inbox as a
+/// `Sender<Envelope<G>>` (via [`register_with`]), and the server's `call` /
+/// `cast` / `whereis_server` recover that exact typed sender here to rebuild a
+/// `GenServerRef<G>`. `None` if unbound, dead (pruned on the way out), or
+/// holding no `M` channel.
 pub(crate) fn resolve_named_sender<M: Send + 'static>(name: &str) -> Option<(Pid, Sender<M>)> {
     with_runtime(|inner| {
         let mut reg = inner.registry.lock();
-        let idx = *reg.by_name.get(name)?;
-        let pid = match reg.by_index.get(&idx).map(|m| m.pid) {
-            Some(pid) if live(inner, pid) => pid,
-            Some(_) => {
-                reg.prune(idx);
-                return None;
-            }
-            None => {
-                reg.by_name.remove(name);
-                return None;
-            }
-        };
-        let tx = reg.by_index.get(&idx).and_then(Mailbox::clone_sender::<M>)?;
+        let pid = *reg.by_name.get(name)?;
+        if !live(inner, pid) {
+            // Stored-pid liveness, generation included: a name whose holder
+            // died is pruned (heals) even if the slot has a new tenant.
+            // Otherwise the tenant's mailbox would make the name unresolvable
+            // without pruning, wedging it for the tenant's lifetime.
+            reg.prune_holder(pid);
+            return None;
+        }
+        // A live holder's mailbox is its own (publish replaces wholesale on
+        // pid mismatch, and one live actor per slot), so index lookup is safe.
+        let tx = reg.by_index.get(&pid.index()).and_then(Mailbox::clone_sender::<M>)?;
         Some((pid, tx))
     })
 }
 
-/// Remove the binding for `name`, returning the actor it pointed at if still
-/// live. Only the *name* is freed; the actor's mailbox (and any other names for
-/// it) remain. A binding to a dead actor is reported as `None`.
+/// Give up a name. Returns the actor it pointed at, if that actor was still
+/// live. Only the *name* is freed; the actor's mailbox (and any other names
+/// bound to it) are unaffected. A binding to an already-dead actor reports
+/// `None`, since there was nothing live to release.
 pub fn unregister(name: &str) -> Option<Pid> {
     with_runtime(|inner| {
         let mut reg = inner.registry.lock();
-        let idx = reg.by_name.remove(name)?;
-        match reg.by_index.get(&idx).map(|m| m.pid) {
-            Some(pid) if live(inner, pid) => Some(pid),
-            _ => None,
-        }
+        let pid = reg.by_name.remove(name)?;
+        if live(inner, pid) { Some(pid) } else { None }
     })
 }
 
-/// Resolve `name` to its actor's `Sender<M>` and deliver `msg`. The whole point
-/// of the rework: a name you can *send* to.
+/// Look `name` up and deliver `msg` to whichever actor currently holds it.
+/// This is the point of naming an actor: a name you can send a message to
+/// directly, without a separate lookup step.
 ///
-/// Errors (message returned in every case): [`SendError::Unresolved`] if no
-/// live actor holds the name, [`SendError::NoChannel`] if that actor has no
-/// channel for `M`, [`SendError::Closed`] if its `M` channel's receiver is
-/// gone. Panics if called outside `Runtime::run()`.
+/// On failure the message comes back to you, wrapped in the [`SendError`]
+/// variant that explains why: [`SendError::Unresolved`] if no live actor
+/// currently holds the name, [`SendError::NoChannel`] if the actor that holds
+/// it never published a channel for `M`, or [`SendError::Closed`] if it
+/// published one but has since dropped the receiving end. Panics if called
+/// outside [`run`](crate::run).
 pub fn send<M: Send + 'static>(name: Name<M>, msg: M) -> Result<(), SendError<M>> {
     let key = name.as_str();
     with_runtime(|inner| {
-        // Resolve + clone the sender under the Leaf lock, then drop the lock
-        // before sending (a send can unpark a receiver).
+        // Resolve + clone the sender under the registry lock, then drop the
+        // lock before sending (a send can unpark a receiver).
         let tx = {
             let mut reg = inner.registry.lock();
-            let idx = match reg.by_name.get(key) {
-                Some(&i) => i,
+            let pid = match reg.by_name.get(key) {
+                Some(&p) => p,
                 None => return Err(SendError::Unresolved(msg)),
             };
-            let pid = match reg.by_index.get(&idx).map(|m| m.pid) {
-                Some(pid) => pid,
-                None => {
-                    reg.by_name.remove(key);
-                    return Err(SendError::Unresolved(msg));
-                }
-            };
             if !live(inner, pid) {
-                reg.prune(idx);
+                // Stored-pid liveness (generation included), so a recycled
+                // slot's new live tenant is never mistaken for the name's
+                // original (now-dead) holder.
+                reg.prune_holder(pid);
                 return Err(SendError::Unresolved(msg));
             }
-            match reg.by_index.get(&idx).and_then(Mailbox::clone_sender::<M>) {
+            match reg.by_index.get(&pid.index()).and_then(Mailbox::clone_sender::<M>) {
                 Some(tx) => tx,
                 None => return Err(SendError::NoChannel(msg)),
             }
@@ -501,18 +578,19 @@ pub fn send<M: Send + 'static>(name: Name<M>, msg: M) -> Result<(), SendError<M>
 
 /// Resolve a *raw* pid to its mailbox and deliver `msg` on the channel for `M`,
 /// with **no redirect**. The stored mailbox must be this exact incarnation
-/// (generation included) and still live; otherwise the actor this pid named is
-/// gone and the result is [`SendError::Dead`] — even when the slot now holds a
-/// different, live actor (which we leave untouched). Shared by [`send_to`]
-/// (typed, `M = A::Msg`, channel guaranteed on an installed actor) and
-/// [`send_dyn`] (explicit `M`, where `NoChannel` is a real outcome).
+/// (generation included) and still live; otherwise the actor this pid named
+/// is gone and the result is [`SendError::Dead`], even when the slot now
+/// holds a different, live actor (which is left untouched). Shared by
+/// [`send_to`] (typed, `M = A::Msg`, channel guaranteed on an installed
+/// actor) and [`send_dyn`] (explicit `M`, where `NoChannel` is a real
+/// outcome).
 fn send_to_pid<M: Send + 'static>(
     inner: &crate::runtime::RuntimeInner,
     pid: Pid,
     msg: M,
 ) -> Result<(), SendError<M>> {
-    // Resolve + clone the sender under the Leaf lock, then drop the lock before
-    // sending (a send can unpark a receiver) — Leaf -> Channel, as name `send`.
+    // Resolve + clone the sender under the registry lock, then drop the lock
+    // before sending (a send can unpark a receiver), same order as `send`.
     let tx = {
         let mut reg = inner.registry.lock();
         match reg.by_index.get(&pid.index()).map(|m| m.pid) {
@@ -525,7 +603,7 @@ fn send_to_pid<M: Send + 'static>(
             }
             // Our incarnation's mailbox, but the actor has died: prune + Dead.
             Some(stored) if stored == pid => {
-                reg.prune(pid.index());
+                reg.prune_holder(pid);
                 return Err(SendError::Dead(msg));
             }
             // A different incarnation (or nothing) occupies the slot: the actor
@@ -536,32 +614,36 @@ fn send_to_pid<M: Send + 'static>(
     tx.send(msg).map_err(|crate::channel::SendError(m)| SendError::Closed(m))
 }
 
-/// Deliver `msg` to the exact actor named by `pid` — RFC 014 §4.2's direct,
-/// identity-bound addressing mode. Unlike name-addressed [`send`] there is **no
-/// redirect**: if that incarnation has died the message comes back as
-/// [`SendError::Dead`], even if its slot now holds a different actor.
+/// Deliver `msg` directly to the exact actor identified by `pid`. Unlike
+/// name-addressed [`send`], there is **no redirect**: if that specific actor
+/// has died, the message comes back as [`SendError::Dead`], even if its slot
+/// has since been taken over by a different, live actor. Use this when you
+/// already hold a `Pid<A>` and want to talk to that one actor specifically;
+/// use [`send`] with a [`Name`] when you want whichever actor currently holds
+/// a name.
 ///
 /// The message type is the actor's `A::Msg`, so on a live actor that has
-/// installed its inbox (via [`install`] or [`register`]) the channel is always
-/// present; [`SendError::NoChannel`] therefore means the actor is live but
-/// never published a `Pid<A>`-reachable inbox. Panics if called outside
-/// `Runtime::run()`.
+/// installed its inbox (via [`install`] or [`register`]) the channel is
+/// always present; [`SendError::NoChannel`] therefore means the actor is live
+/// but never published a `Pid<A>`-reachable inbox. Panics if called outside
+/// [`run`](crate::run).
 pub fn send_to<A: Addressable>(pid: Pid<A>, msg: A::Msg) -> Result<(), SendError<A::Msg>> {
     with_runtime(|inner| send_to_pid::<A::Msg>(inner, pid.erase(), msg))
 }
 
-/// The explicit bare-pid escape hatch (RFC 014 §4.6): deliver `msg` of type `M`
-/// to `pid` when all you hold is an untyped [`Pid`] — a pid off a [`Down`], or
-/// out of a future `members()` — so the typed [`send_to`] is unavailable.
+/// The escape hatch for sending to a bare, untyped [`Pid`] when the typed
+/// [`send_to`] is unavailable, for example a pid recovered from a [`Down`]
+/// notification or a group's `members()` list, where you no longer know the
+/// actor's message type at compile time.
 ///
-/// This is the one send whose message type can genuinely be wrong: the actor
-/// may be live yet expose no channel for `M`, returning [`SendError::NoChannel`]
-/// (on the typed paths that downcast collapses to a `debug_assert`). It is
-/// named and documented as the fallible fallback so the typed `Pid<A>` /
-/// `Name<M>` paths stay the obvious default and an agentic caller reaches for a
-/// present primitive instead of inventing a workaround. Liveness is identical
-/// to [`send_to`]: identity-bound, no redirect, [`SendError::Dead`] once the
-/// addressed incarnation is gone. Panics if called outside `Runtime::run()`.
+/// Because the message type is not checked at compile time here, this is the
+/// one send that can genuinely be live-but-wrong: the actor may be alive yet
+/// expose no channel for `M`, in which case you get [`SendError::NoChannel`]
+/// back instead of a misdelivery. Liveness and redirect behavior are
+/// otherwise identical to [`send_to`]: identity-bound, no redirect,
+/// [`SendError::Dead`] once the addressed incarnation is gone. Prefer
+/// `send_to` with a typed `Pid<A>` whenever you have one; reach for this only
+/// when you don't. Panics if called outside [`run`](crate::run).
 ///
 /// [`Down`]: crate::Down
 pub fn send_dyn<M: Send + 'static>(pid: Pid, msg: M) -> Result<(), SendError<M>> {

src/run_queue.rs

@@ -113,16 +113,32 @@ impl MutexQueue {
 
     pub fn push(&self, pid: Pid) {
         assert_no_preempt();
-        self.q.lock().unwrap().push_back(pid);
+        match self.q.lock() {
+            Ok(mut g) => g.push_back(pid),
+            Err(e) => panic!("smarm: run-queue q lock poisoned (core corrupt): {e}"),
+        }
     }
 
     pub fn pop(&self) -> Option<Pid> {
         assert_no_preempt();
-        self.q.lock().unwrap().pop_front()
+        match self.q.lock() {
+            Ok(mut g) => g.pop_front(),
+            Err(e) => panic!("smarm: run-queue q lock poisoned (core corrupt): {e}"),
+        }
     }
 
     pub fn len(&self) -> u64 {
-        self.q.lock().unwrap().len() as u64
+        match self.q.lock() {
+            Ok(g) => g.len() as u64,
+            Err(e) => panic!("smarm: run-queue q lock poisoned (core corrupt): {e}"),
+        }
+    }
+
+    pub fn is_empty(&self) -> bool {
+        match self.q.lock() {
+            Ok(g) => g.is_empty(),
+            Err(e) => panic!("smarm: run-queue q lock poisoned (core corrupt): {e}"),
+        }
     }
 }
 
@@ -262,6 +278,10 @@ impl MpmcRing {
         let d = self.dequeue_pos.0.load(Ordering::Relaxed);
         e.saturating_sub(d) as u64
     }
+
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
 }
 
 // ---------------------------------------------------------------------------
@@ -341,6 +361,10 @@ impl StripedRing {
     pub fn len(&self) -> u64 {
         self.stripes.iter().map(|s| s.len()).sum()
     }
+
+    pub fn is_empty(&self) -> bool {
+        self.stripes.iter().all(|s| s.is_empty())
+    }
 }
 
 // ---------------------------------------------------------------------------

src/runtime.rs

@@ -86,21 +86,28 @@
 //! # Termination (counter-based)
 //!
 //! The old all-clear scanned the slot table under the big lock. Now:
-//! exit when `io_out == 0` (read *before* the queue lock, phase-1 ordering)
-//! and, under the queue lock, the queue is empty and `live_actors == 0`.
-//! `live_actors` is incremented in `spawn` before the enqueue and decremented
-//! at the very END of `finalize_actor`, strictly after every wakeup that
-//! finalize produces has been enqueued. The soundness crux: any enqueue
-//! targets a live (not-yet-finalized) actor, so `live == 0` implies no wakeup
-//! can still be in flight; combined with "spawner is itself live", observing
-//! `(queue empty, live == 0)` under the queue lock means no work can ever
-//! appear again.
+//! exit when `io_outstanding + io_fd_waiters == 0` (two Relaxed/Acquire
+//! atomic loads, read *before* the queue pop) and, under the queue lock,
+//! the queue is empty and `live_actors == 0`. `live_actors` is incremented
+//! in `spawn` before the enqueue and decremented at the very END of
+//! `finalize_actor`, strictly after every wakeup that finalize produces has
+//! been enqueued. The soundness crux: any enqueue targets a live
+//! (not-yet-finalized) actor, so `live == 0` implies no wakeup can still be
+//! in flight; combined with "spawner is itself live", observing
+//! `(queue empty, live == 0)` means no work can ever appear again.
 //!
-//! # Timer / IO drain (try-lock, one-winner)
+//! # Scheduler park/wake (RFC 018)
 //!
-//! Unchanged from phase 1: one winner per round drains due timers and IO
-//! completions from their own mutexes; wakeups go through the unpark
-//! protocol like everyone else's.
+//! Schedulers sleep on per-thread futex parkers via the coordination layer
+//! (`park.rs`), NOT on a shared wake pipe. IO backends are producers behind
+//! a two-call contract — make the actor runnable (`unpark_at`), whose
+//! `enqueue` tail wakes exactly one parked scheduler. The blocking pool and
+//! epoll thread each route their own completions (driver-enqueues); there
+//! is no shared completion queue, no drain lock, no one-winner drain phase.
+//! Timers fire two ways: a busy-path due-check every loop iteration (one
+//! Relaxed load of the earliest-deadline snapshot when no timer is armed),
+//! and the timekeeper — at most one parked scheduler holds the timer
+//! deadline, so an expiry wakes one scheduler, not a herd.
 
 use crate::actor::{
     clear_current_pid, is_actor_done, reset_actor_done, set_current_actor_box,
@@ -445,6 +452,42 @@ pub(crate) struct Slot {
     /// and only when the `budget-accounting` feature is on (it costs two RDTSC
     /// per resume); stays 0 otherwise. Same single-writer Relaxed discipline.
     budget_cycles: AtomicU64,
+    /// RFC 007 (`smarm-causal`) — id of the causal-profiling site this actor
+    /// is currently inside (0 = none). Lives in the slot, not a thread-local,
+    /// so it survives preemption and cross-scheduler migration. Written only
+    /// by the actor itself (guard enter/exit on its own thread), read by that
+    /// same thread in `maybe_preempt` — single-writer Relaxed, like `overruns`.
+    /// Exists regardless of the feature; stays 0 without it.
+    causal_site: AtomicU32,
+    /// RFC 007 (`smarm-causal`) — virtual-speedup delay cycles this actor has
+    /// absorbed (or been credited). Compared against the global ledger in
+    /// `causal::check`; fast-forwarded on resume-from-park so blocked time
+    /// absorbs delay for free (Coz's blocked-thread rule). Same single-writer
+    /// discipline.
+    causal_delay: AtomicU64,
+    /// RFC 007 (`smarm-causal`) — true iff this actor's last deschedule was a
+    /// real park (a successful `park_return`, not a slice yield and not the
+    /// instant-wake re-queue). Consumed by `causal::on_resume`: only a wake
+    /// from genuine blocking forgives outstanding virtual delay; a merely
+    /// preempted (runnable) actor stays in debt and must pay by spinning.
+    /// Starts false: a fresh spawn is born current (`reset_counters` sets
+    /// `causal_delay` to the global ledger), and anything injected while it
+    /// sits spawn-queued is owed, not waived. Written by the owning
+    /// scheduler thread at the deschedule /
+    /// resume boundary only — same single-writer discipline as `causal_delay`.
+    causal_parked: AtomicBool,
+    /// RFC 007 — TSC at this actor's last *runnable* (yield) deschedule
+    /// while it sat in the live experiment's target site; 0 = no gap
+    /// pending. Paired with `causal_desched_epoch`; written on the
+    /// deschedule path, consumed at the next resume — same single-writer
+    /// discipline as `causal_delay`.
+    causal_desched_tsc: AtomicU64,
+    /// RFC 007 — experiment epoch live at that deschedule (see
+    /// `causal::EXPERIMENT_EPOCH`): the resume counts the gap only into
+    /// the same window, so one straddling `end()` — or a later `begin()`
+    /// with an identical site+pct word — is dropped instead of leaking a
+    /// cooldown across windows.
+    causal_desched_epoch: AtomicU64,
     /// Cold lifecycle data. See [`SlotCold`].
     pub(crate) cold: RawMutex<SlotCold>,
 }
@@ -459,6 +502,14 @@ impl Slot {
             overruns: AtomicU64::new(0),
             messages_received: AtomicU64::new(0),
             budget_cycles: AtomicU64::new(0),
+            causal_site: AtomicU32::new(0),
+            causal_delay: AtomicU64::new(0),
+            // Overwritten at every occupancy by `reset_counters` (born
+            // current); false so a hypothetical reset-skipping path cannot
+            // waive the whole global backlog at first resume.
+            causal_parked: AtomicBool::new(false),
+            causal_desched_tsc: AtomicU64::new(0),
+            causal_desched_epoch: AtomicU64::new(0),
             cold: RawMutex::new(SlotCold {
                 actor: None,
                 waiters: Vec::new(),
@@ -547,6 +598,92 @@ impl Slot {
         self.overruns.store(0, Ordering::Relaxed);
         self.messages_received.store(0, Ordering::Relaxed);
         self.budget_cycles.store(0, Ordering::Relaxed);
+        self.causal_site.store(0, Ordering::Relaxed);
+        // Born current (Coz's new-thread rule): a fresh incarnation neither
+        // owes the process's accumulated delay history nor books it as park
+        // forgiveness. The old init (delay 0, parked true) waived the whole
+        // monotone backlog once per spawn via the first resume — found live
+        // under close-mode conn churn (~95k spawns/s): millions of phantom
+        // forgiven ms per 700ms window, even in 0% cells. Parked starts
+        // false: delay injected while spawn-queued is *owed* (the newborn is
+        // runnable, not blocked) and paid at its first check — the semantics
+        // `audit_zero_pct_window_absorbs_leftover_debt` pins.
+        #[cfg(feature = "smarm-causal")]
+        self.causal_delay
+            .store(crate::causal::global_delay_cycles(), Ordering::Relaxed);
+        #[cfg(not(feature = "smarm-causal"))]
+        self.causal_delay.store(0, Ordering::Relaxed);
+        self.causal_parked.store(false, Ordering::Relaxed);
+        self.causal_desched_tsc.store(0, Ordering::Relaxed);
+        self.causal_desched_epoch.store(0, Ordering::Relaxed);
+    }
+
+    /// RFC 007 — mark that this actor's deschedule was a genuine park.
+    /// Called from the scheduler's `YieldIntent::Park` branch on a successful
+    /// `park_return` only.
+    #[cfg_attr(not(feature = "smarm-causal"), allow(dead_code))]
+    #[inline]
+    pub(crate) fn set_causal_parked(&self) {
+        self.causal_parked.store(true, Ordering::Relaxed);
+    }
+
+    /// RFC 007 — consume the parked marker at resume: returns whether the
+    /// last deschedule was a real park, and clears it so the next resume
+    /// defaults to "was runnable" unless the park branch says otherwise.
+    #[cfg_attr(not(feature = "smarm-causal"), allow(dead_code))]
+    #[inline]
+    pub(crate) fn take_causal_parked(&self) -> bool {
+        self.causal_parked.swap(false, Ordering::Relaxed)
+    }
+
+    /// RFC 007 — stash the runnable-deschedule instant and the experiment
+    /// epoch it happened under (offcpu-gap audit; see `causal::on_resume`).
+    #[cfg_attr(not(feature = "smarm-causal"), allow(dead_code))]
+    #[inline]
+    pub(crate) fn set_causal_desched(&self, tsc: u64, epoch: u64) {
+        self.causal_desched_tsc.store(tsc, Ordering::Relaxed);
+        self.causal_desched_epoch.store(epoch, Ordering::Relaxed);
+    }
+
+    /// RFC 007 — consume the stash: `(tsc, epoch)`; `(0, _)` = none pending.
+    #[cfg_attr(not(feature = "smarm-causal"), allow(dead_code))]
+    #[inline]
+    pub(crate) fn take_causal_desched(&self) -> (u64, u64) {
+        let tsc = self.causal_desched_tsc.swap(0, Ordering::Relaxed);
+        let epoch = self.causal_desched_epoch.load(Ordering::Relaxed);
+        (tsc, epoch)
+    }
+
+    /// RFC 007 — current causal site id (0 = none). Single-writer: only the
+    /// on-CPU actor's thread writes, via the site-guard enter/exit.
+    #[cfg_attr(not(feature = "smarm-causal"), allow(dead_code))]
+    #[inline]
+    pub(crate) fn causal_site(&self) -> u32 {
+        self.causal_site.load(Ordering::Relaxed)
+    }
+
+    /// RFC 007 — write the current causal site id (guard enter/exit).
+    #[cfg_attr(not(feature = "smarm-causal"), allow(dead_code))]
+    #[inline]
+    pub(crate) fn set_causal_site(&self, id: u32) {
+        self.causal_site.store(id, Ordering::Relaxed);
+    }
+
+    /// RFC 007 — absorbed/credited virtual-delay cycles.
+    #[cfg_attr(not(feature = "smarm-causal"), allow(dead_code))]
+    #[inline]
+    pub(crate) fn causal_delay(&self) -> u64 {
+        self.causal_delay.load(Ordering::Relaxed)
+    }
+
+    /// RFC 007 — set the absorbed-delay ledger (spin-absorb, credit, or the
+    /// resume-path fast-forward). Single-writer per the resume protocol: the
+    /// actor's own thread while on-CPU, the resuming scheduler thread at the
+    /// resume boundary — never both at once.
+    #[cfg_attr(not(feature = "smarm-causal"), allow(dead_code))]
+    #[inline]
+    pub(crate) fn set_causal_delay(&self, v: u64) {
+        self.causal_delay.store(v, Ordering::Relaxed);
     }
 
     /// A pid's-eye snapshot of the slot. Cold paths re-read this under the
@@ -620,8 +757,21 @@ pub(crate) struct RuntimeInner {
     pub(crate) io: Mutex<Option<IoThread>>,
     /// Monotonic `MonitorId` source. Never reused.
     pub(crate) next_monitor_id: AtomicU64,
-    /// Try-lock: exactly one scheduler thread drains timers/IO per iteration.
-    drain_lock: Mutex<()>,
+    /// RFC 018: the scheduler coordination layer — per-scheduler parkers,
+    /// idle mask, wake protocol, timekeeper role, earliest-deadline
+    /// snapshot. Arc'd because `Timers` shares it (insert-side deadline
+    /// notes run under the timers mutex).
+    pub(crate) coord: Arc<crate::park::Coordinator>,
+    /// `block_on_io` requests in flight. Incremented by the submitter
+    /// BEFORE submit (underflow-proof), decremented by the pool thread on
+    /// completion. Read lock-free by the idle path's termination verdict —
+    /// the per-pop `io.lock` of the drain era is gone.
+    pub(crate) io_outstanding: AtomicU32,
+    /// Parked fd waiters. Incremented by the registrar BEFORE
+    /// `epoll_register` (rolled back on error), decremented by whoever
+    /// consumes the registration (epoll thread on readiness, canceller on
+    /// an unwound wait). Same lock-free verdict read as `io_outstanding`.
+    pub(crate) io_fd_waiters: AtomicU32,
     /// Per-thread stats, indexed by scheduler thread slot (0..N).
     pub(crate) stats: Vec<SchedulerStats>,
     /// Global counters for RFC 000 primitives.
@@ -655,6 +805,9 @@ pub(crate) struct RuntimeInner {
 }
 
 impl RuntimeInner {
+    // Private constructor taking the parsed Config fields one-for-one; a params
+    // struct would only move the same 8 values across the call boundary.
+    #[allow(clippy::too_many_arguments)]
     fn new(
         thread_count: usize,
         alloc_interval: u32,
@@ -669,6 +822,12 @@ impl RuntimeInner {
         let slots: Box<[Slot]> = (0..max_actors).map(|_| Slot::vacant()).collect();
         // Low indices on top of the stack so early spawns get low pids.
         let free: Vec<u32> = (0..max_actors as u32).rev().collect();
+        // RFC 018: the coordination layer (asserts thread_count <= 64), and
+        // the timers' hook into it — every insert under the timers mutex
+        // notes its deadline (busy-path snapshot + timekeeper re-arm).
+        let coord = Arc::new(crate::park::Coordinator::new(thread_count));
+        let mut timers = Timers::new();
+        timers.attach_coordinator(coord.clone());
         Arc::new(Self {
             run_queue: crate::run_queue::RunQueue::new(thread_count, max_actors),
             slots,
@@ -677,10 +836,12 @@ impl RuntimeInner {
             root_bits: AtomicU64::new(u64::MAX),
             root_exited: AtomicBool::new(false),
             root_swept: AtomicBool::new(false),
-            timers: Mutex::new(Timers::new()),
+            timers: Mutex::new(timers),
             io: Mutex::new(None),
             next_monitor_id: AtomicU64::new(0),
-            drain_lock: Mutex::new(()),
+            coord,
+            io_outstanding: AtomicU32::new(0),
+            io_fd_waiters: AtomicU32::new(0),
             stats,
             io_parked: AtomicU32::new(0),
             sleeping: AtomicU32::new(0),
@@ -741,6 +902,13 @@ impl RuntimeInner {
         );
         self.run_queue.push(pid);
         crate::te!(crate::trace::Event::Enqueue(pid));
+        // RFC 018 enqueue wake (fixes the silent enqueue): if a scheduler
+        // is parked, wake exactly one. The fast path when everyone is busy
+        // is a fence + one Relaxed load of an unmodified line — the
+        // pure-compute hot path pays (almost) nothing. Bias is over-wake:
+        // a spurious wake costs one futex round-trip and a failed pop; a
+        // missed wake would cost a stranded actor.
+        self.coord.wake_one_if_idle();
     }
 
     /// Make `pid` runnable if it is parked; coalesce or defer otherwise.
@@ -762,7 +930,10 @@ impl RuntimeInner {
     /// park-epoch and return it. See slot_state.rs for the rules.
     #[must_use]
     pub(crate) fn begin_wait(&self, pid: Pid) -> u32 {
-        let slot = self.slot_at(pid).expect("begin_wait: own slot vanished");
+        let slot = match self.slot_at(pid) {
+            Some(slot) => slot,
+            None => panic!("begin_wait: own slot vanished: {:?}", pid),
+        };
         slot.word.begin_wait(pid.generation())
     }
 
@@ -771,7 +942,10 @@ impl RuntimeInner {
     /// then eat a notification that already landed. The caller MUST
     /// re-check its stop flag afterwards — see `StateWord::clear_notify`.
     pub(crate) fn retire_wait(&self, pid: Pid) {
-        let slot = self.slot_at(pid).expect("retire_wait: own slot vanished");
+        let slot = match self.slot_at(pid) {
+            Some(slot) => slot,
+            None => panic!("retire_wait: own slot vanished: {:?}", pid),
+        };
         let _ = slot.word.begin_wait(pid.generation());
         slot.word.clear_notify(pid.generation());
     }
@@ -890,6 +1064,11 @@ pub fn init(config: Config) -> Runtime {
 impl Runtime {
     /// Run `f` as the initial actor, block until all actors finish.
     /// Can be called multiple times sequentially on the same `Runtime`.
+    ///
+    /// A panic in the initial actor propagates out of `run` (re-raised
+    /// after teardown, so the `Runtime` stays reusable even under a
+    /// caller's `catch_unwind`). Panics in other actors surface through
+    /// joins, links, monitors, and supervisors as usual.
     pub fn run(&self, f: impl FnOnce() + Send + 'static) {
         // Install smarm's panic hook on first call. The default Rust hook is
         // not reentrant — concurrent actor panics can trigger a double-panic
@@ -932,7 +1111,23 @@ impl Runtime {
             self.inner.live_actors.load(Ordering::Acquire), 0,
             "run() called while previous run still active"
         );
-        *self.inner.io.lock().unwrap() = Some(IoThread::start().expect("failed to start IO thread"));
+        // RFC 018: the IO producers reach the runtime (slot table + unpark)
+        // through a Weak, so no RuntimeInner → IoThread → RuntimeInner cycle
+        // forms. Reset the in-flight counters BEFORE the threads can touch
+        // them (a prior run left them at 0 on a clean exit; the asserts pin
+        // that).
+        debug_assert_eq!(self.inner.io_outstanding.load(Ordering::Acquire), 0);
+        debug_assert_eq!(self.inner.io_fd_waiters.load(Ordering::Acquire), 0);
+        self.inner.io_outstanding.store(0, Ordering::Release);
+        self.inner.io_fd_waiters.store(0, Ordering::Release);
+        let io_thread = match IoThread::start(Arc::downgrade(&self.inner)) {
+            Ok(io) => io,
+            Err(e) => panic!("failed to start IO thread: {e}"),
+        };
+        match self.inner.io.lock() {
+            Ok(mut io) => *io = Some(io_thread),
+            Err(e) => panic!("smarm: io lock poisoned (core corrupt): {e}"),
+        }
 
         // RFC 005: slot counters reset at the START of a run (not the end),
         // so `stats()` read after `run()` returns reports that run's totals.
@@ -951,16 +1146,27 @@ impl Runtime {
         self.inner.root_swept.store(false, Ordering::Relaxed);
         self.inner.set_root(initial_handle.pid());
 
-        // Launch N-1 extra scheduler threads. The calling thread is thread 0.
+        // Launch N-1 extra scheduler threads, named `smarm-sched-{slot}` so
+        // they are identifiable in `/proc/<pid>/task/*/comm`, stack dumps and
+        // debuggers. The calling thread is thread 0 and keeps its caller-given
+        // name (an embedder typically names it when spawning `run`).
         let mut os_threads = Vec::new();
         for slot in 1..self.thread_count {
             let inner = self.inner.clone();
-            let t = thread::spawn(move || {
-                RUNTIME.with(|r| *r.borrow_mut() = Some(inner.clone()));
-                SCHED_SLOT.with(|s| s.set(slot));
-                schedule_loop(&inner, slot);
-                RUNTIME.with(|r| *r.borrow_mut() = None);
-            });
+            let t = thread::Builder::new()
+                .name(format!("smarm-sched-{slot}"))
+                .spawn(move || {
+                    RUNTIME.with(|r| *r.borrow_mut() = Some(inner.clone()));
+                    SCHED_SLOT.with(|s| s.set(slot));
+                    schedule_loop(&inner, slot);
+                    RUNTIME.with(|r| *r.borrow_mut() = None);
+                });
+            // `thread::spawn` (the previous form) also panics when the OS
+            // refuses a thread, so this keeps the failure semantics.
+            let t = match t {
+                Ok(t) => t,
+                Err(e) => panic!("failed to spawn smarm scheduler thread: {e}"),
+            };
             os_threads.push(t);
         }
 
@@ -973,12 +1179,35 @@ impl Runtime {
             let _ = t.join();
         }
 
+        // The root's outcome, read before its handle drops (the outstanding
+        // handle pins the slot: no reclaim, no generation change under us).
+        // A root panic must escape `run()` — propagated at the tail below,
+        // after teardown. Dropping it unread here made every assert inside
+        // `run` silently vacuous (found live: a failing-first test passed).
+        let root_outcome = {
+            let slot = match self.inner.slot_at(initial_handle.pid()) {
+                Some(slot) => slot,
+                None => panic!("run(): root pid out of range"),
+            };
+            debug_assert!(
+                slot.status_for(initial_handle.pid()) == Status::Done,
+                "root not Done at run() teardown"
+            );
+            slot.cold.lock().outcome.take()
+        };
+
         // Drop initial handle (decrements outstanding_handles count).
         drop(initial_handle);
 
         // Tear down IO and clean up for the next run() call.
-        drop(self.inner.io.lock().unwrap().take()); // joins IO threads
-        self.inner.timers.lock().unwrap().clear();
+        match self.inner.io.lock() {
+            Ok(mut io) => drop(io.take()), // joins IO threads
+            Err(e) => panic!("smarm: io lock poisoned (core corrupt): {e}"),
+        }
+        match self.inner.timers.lock() {
+            Ok(mut timers) => timers.clear(),
+            Err(e) => panic!("smarm: timers lock poisoned (core corrupt): {e}"),
+        }
         self.inner.next_monitor_id.store(0, Ordering::Relaxed);
         // Every slot must have come back: any leak here is a runtime bug
         // (a JoinHandle held across run() is decremented just above).
@@ -994,12 +1223,34 @@ impl Runtime {
         }
         self.inner.io_parked.store(0, Ordering::Relaxed);
         self.inner.sleeping.store(0, Ordering::Relaxed);
+        self.inner.io_outstanding.store(0, Ordering::Relaxed);
+        self.inner.io_fd_waiters.store(0, Ordering::Relaxed);
 
         RUNTIME.with(|r| *r.borrow_mut() = None);
 
         // Flush trace to disk (no-op without smarm-trace).
         #[cfg(feature = "smarm-trace")]
         crate::trace::flush();
+
+        // Propagate a root panic — last, after the full teardown above, so
+        // a `catch_unwind` around `run()` leaves the `Runtime` reusable.
+        // `Exit` and `Stopped` return normally: a cooperatively stopped
+        // root is not an error. `resume_unwind` re-raises the original
+        // payload; the throw-site hook output was suppressed in-actor, so
+        // the caller sees the payload message without the origin file:line.
+        if let Some(Outcome::Panic(payload)) = root_outcome {
+            // Surface the message: the throw-site hook output was suppressed
+            // in-actor, so without this a harness shows a bare FAILED.
+            let msg: Option<&str> = payload
+                .downcast_ref::<&'static str>()
+                .copied()
+                .or_else(|| payload.downcast_ref::<String>().map(String::as_str));
+            eprintln!(
+                "smarm: root actor panicked: {}",
+                msg.unwrap_or("<non-string panic payload>")
+            );
+            std::panic::resume_unwind(payload);
+        }
     }
 
     /// Snapshot of runtime statistics for introspection / tests.
@@ -1156,10 +1407,16 @@ fn finalize_actor(inner: &Arc<RuntimeInner>, pid: Pid, outcome: Outcome) {
         Outcome::Stopped => (Outcome::Stopped, Signal::Stopped(pid), DownReason::Stopped),
     };
 
-    let slot = inner.slot_at(pid).expect("finalize_actor: pid out of range");
+    let slot = match inner.slot_at(pid) {
+        Some(slot) => slot,
+        None => panic!("finalize_actor: pid out of range: {:?}", pid),
+    };
     let (waiters, monitors, links, actor) = {
         let mut cold = slot.cold.lock();
-        let actor = cold.actor.take().expect("finalize_actor: actor vanished");
+        let actor = match cold.actor.take() {
+            Some(actor) => actor,
+            None => panic!("finalize_actor: actor vanished"),
+        };
         cold.outcome = Some(joiner_outcome);
         slot.stop_ptr.store(std::ptr::null_mut(), Ordering::Release);
         // Done is published under the cold lock, so join's
@@ -1282,6 +1539,56 @@ fn stop_live_actors(inner: &Arc<RuntimeInner>) {
     }
 }
 
+// ---------------------------------------------------------------------------
+// Timer firing — shared by the busy-path due-check and the timekeeper
+// ---------------------------------------------------------------------------
+
+/// Pop and dispatch every due timer. `pop_due` re-anchors the
+/// earliest-deadline snapshot under the timers mutex before returning, so
+/// a caller that raced a concurrent insert simply comes back on the next
+/// due-check. Dispatch runs with the timers lock released.
+fn fire_due_timers(inner: &Arc<RuntimeInner>, try_only: bool) {
+    let due = if try_only {
+        // Busy path: if another scheduler is already in the timers mutex
+        // (firing, inserting, or peeking) skip — the snapshot stays due
+        // until someone actually pops, so the check re-fires next loop.
+        match inner.timers.try_lock() {
+            Ok(mut t) => t.pop_due(std::time::Instant::now()),
+            Err(std::sync::TryLockError::WouldBlock) => return,
+            Err(std::sync::TryLockError::Poisoned(e)) => {
+                panic!("smarm: timers lock poisoned (core corrupt): {e}")
+            }
+        }
+    } else {
+        match inner.timers.lock() {
+            Ok(mut t) => t.pop_due(std::time::Instant::now()),
+            Err(e) => panic!("smarm: timers lock poisoned (core corrupt): {e}"),
+        }
+    };
+    for entry in due {
+        match entry.reason {
+            // A sleep expiry is just an unpark: the protocol handles
+            // every interleaving — Parked (re-queue), Running (the
+            // actor is between `timers.insert_sleep` and
+            // `park_current`; RunningNotified makes the upcoming park
+            // re-queue), or gone (no-op).
+            crate::timer::Reason::Sleep { epoch } => inner.unpark_at(entry.pid, epoch),
+            crate::timer::Reason::WaitTimeout { target, epoch } => {
+                // The callback may call unpark_at itself.
+                target.on_timeout(entry.pid, epoch);
+            }
+            // A `send_after` deadline: run the captured delivery thunk.
+            // It resolves the destination through the registry and
+            // sends now (a send can unpark a receiver) — same as any
+            // other in-loop unpark. The timers lock is already
+            // released; lock order Leaf -> Channel is preserved by the
+            // send itself. `pop_due` only returns still-armed Sends, so
+            // a cancelled one never reaches here.
+            crate::timer::Reason::Send { fire } => fire(),
+        }
+    }
+}
+
 // ---------------------------------------------------------------------------
 // schedule_loop — runs on each scheduler OS thread
 // ---------------------------------------------------------------------------
@@ -1292,89 +1599,14 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
 
     loop {
         // ----------------------------------------------------------------
-        // 1. Try to win the drain lock (timers + IO). One winner per round;
-        //    losers skip immediately and proceed to step 2.
+        // 1. Busy-path timer due-check (RFC 018 design point (a)): under
+        //    saturation nobody parks, so no timekeeper exists — due timers
+        //    must still fire. One Relaxed load + branch when no timer is
+        //    armed; the clock is read only when one is.
         // ----------------------------------------------------------------
-        if let Ok(_drain_guard) = inner.drain_lock.try_lock() {
-            // Timers and IO live behind their own mutexes (phase 1), so the
-            // pure-yield / pure-compute hot path never contends a global lock
-            // just to discover there is nothing to drain. The clock is read
-            // only when the timer heap is non-empty.
-            let due = {
-                let mut t = inner.timers.lock().unwrap();
-                if t.is_empty() {
-                    Vec::new()
-                } else {
-                    t.pop_due(std::time::Instant::now())
-                }
-            };
-            let completions = inner.io.lock().unwrap()
-                .as_mut()
-                .map(|io| io.drain_completions())
-                .unwrap_or_default();
-            for entry in due {
-                match entry.reason {
-                    // A sleep expiry is just an unpark: the protocol handles
-                    // every interleaving — Parked (re-queue), Running (the
-                    // actor is between `timers.insert_sleep` and
-                    // `park_current`; RunningNotified makes the upcoming park
-                    // re-queue), or gone (no-op).
-                    crate::timer::Reason::Sleep { epoch } => {
-                        inner.unpark_at(entry.pid, epoch)
-                    }
-                    crate::timer::Reason::WaitTimeout { target, epoch } => {
-                        // The callback may call unpark_at itself.
-                        target.on_timeout(entry.pid, epoch);
-                    }
-                    // A `send_after` deadline: run the captured delivery thunk.
-                    // It resolves the destination through the registry and
-                    // sends now (a send can unpark a receiver) — same as any
-                    // other in-loop unpark. The timers lock is already
-                    // released; lock order Leaf -> Channel is preserved by the
-                    // send itself. `pop_due` only returns still-armed Sends, so
-                    // a cancelled one never reaches here.
-                    crate::timer::Reason::Send { fire } => fire(),
-                }
-            }
-
-            for completion in completions {
-                match completion {
-                    crate::io::Completion::Blocking { pid, epoch, result } => {
-                        if let Some(io) = inner.io.lock().unwrap().as_mut() {
-                            io.outstanding = io.outstanding.saturating_sub(1);
-                        }
-                        // Stash the result under the cold lock, then unpark.
-                        // The protocol also covers the submit→park window
-                        // (RunningNotified), which the old code missed for
-                        // Blocking completions — a latent lost wakeup.
-                        if let Some(slot) = inner.slot_at(pid) {
-                            {
-                                let mut cold = slot.cold.lock();
-                                if slot.generation() == pid.generation() {
-                                    cold.pending_io_result = Some(result);
-                                } else {
-                                    // Actor died (stopped) with the op in
-                                    // flight; discard the result.
-                                }
-                            }
-                            inner.unpark_at(pid, epoch);
-                        }
-                    }
-                    crate::io::Completion::FdReady { fd, events: _ } => {
-                        // Resolve the parked pid under the io lock, then wake
-                        // through the protocol. Lock order: io before all.
-                        let parked = inner.io.lock().unwrap().as_mut().and_then(|io| {
-                            let entry = io.waiters.remove(&fd);
-                            io.epoll_deregister(fd);
-                            entry
-                        });
-                        if let Some((pid, epoch)) = parked {
-                            inner.unpark_at(pid, epoch);
-                        }
-                    }
-                }
-            }
-        } // drain_guard drops here
+        if inner.coord.deadline_due() {
+            fire_due_timers(inner, true);
+        }
 
         // ----------------------------------------------------------------
         // 2. Pop a runnable pid. Pop order (RFC 005): wake slot first, then
@@ -1383,7 +1615,7 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
         // ----------------------------------------------------------------
         enum Pop {
             Got(Pid),
-            Idle { io_outstanding: u32, wake_fd: Option<std::os::fd::RawFd> },
+            Idle,
             AllDone,
             /// Root has exited and nothing is runnable: stop the parked-forever
             /// remainder, then re-pop. Fires at most once per run.
@@ -1408,13 +1640,12 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
             crate::te!(crate::trace::Event::SlotPop(pid));
             pid
         } else {
-            // Read IO liveness BEFORE the queue lock (phase-1 ordering: a
-            // completion resurrects an actor only via the drain path, whose
-            // enqueue would be visible under the queue lock we take next).
-            let (io_out, io_fd) = match inner.io.lock().unwrap().as_ref() {
-                Some(io) => (io.outstanding + io.waiters.len() as u32, Some(io.wake_fd())),
-                None => (0, None),
-            };
+            // Read IO liveness BEFORE the queue pop — two atomic loads now
+            // (RFC 018), not a per-pop `io.lock`: a completion resurrects
+            // an actor via the producer's own unpark→enqueue, whose entry
+            // would be visible to the pop below.
+            let io_out = inner.io_outstanding.load(Ordering::Acquire)
+                + inner.io_fd_waiters.load(Ordering::Acquire);
 
             stats.run_queue_len.store(inner.run_queue.len(), Ordering::Relaxed);
             let pop = match inner.run_queue.pop() {
@@ -1446,7 +1677,7 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
                         // the idle wait below on the next pass.
                         Pop::RootDrain
                     } else {
-                        Pop::Idle { io_outstanding: io_out, wake_fd: io_fd }
+                        Pop::Idle
                     }
                 }
             };
@@ -1457,18 +1688,19 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
                     // Remaining timer entries are orphaned (no live actor can be
                     // woken by them — e.g. a sleeper cancelled out of its sleep);
                     // they must not keep the runtime alive. Drop them on the way out.
-                    inner.timers.lock().unwrap().clear();
-                    // Terminal wake: a sibling scheduler may be blocked in its
-                    // idle wait on a snapshot that is now terminally stale — an
-                    // orphaned long deadline (it would sleep it out in full) or
-                    // a stale `io_outstanding > 0` from a stop-cancelled waiter
-                    // (it would block in poll(-1) forever; cancellation produces
-                    // no completion, so nothing else writes the wake pipe).
-                    // One byte wakes every poller; each re-runs the verdict,
-                    // reaches AllDone itself, and re-wakes — idempotent.
-                    if let Some(io) = inner.io.lock().unwrap().as_ref() {
-                        io.wake();
+                    match inner.timers.lock() {
+                        Ok(mut timers) => timers.clear(),
+                        Err(e) => panic!("smarm: timers lock poisoned (core corrupt): {e}"),
                     }
+                    // Terminal wake (replaces the wake-pipe byte): a sibling
+                    // may be parked on a snapshot that is now terminally
+                    // stale — an orphaned long deadline, or a stale
+                    // `io_fd_waiters > 0` from a stop-cancelled waiter
+                    // (cancellation produces no completion, so nothing else
+                    // will ever wake it). `wake_all` permits every parker;
+                    // each sibling re-runs the verdict, reaches AllDone
+                    // itself, and re-wakes — idempotent.
+                    inner.coord.wake_all();
                     return;
                 }
                 Pop::RootDrain => {
@@ -1478,31 +1710,51 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
                     stop_live_actors(inner);
                     continue;
                 }
-                Pop::Idle { io_outstanding, wake_fd } => {
-                    // Something is still in flight. Sleep on the appropriate
-                    // source to avoid hammering the queue mutex; retry on wake.
-                    let next_deadline = inner.timers.lock().unwrap().peek_deadline();
-                    match (next_deadline, wake_fd) {
-                        (Some(deadline), fd_opt) => {
-                            let now = std::time::Instant::now();
-                            if deadline > now {
-                                let timeout = deadline - now;
-                                match fd_opt {
-                                    Some(fd) => {
-                                        crate::io::poll_wake(fd, Some(timeout));
-                                        crate::io::drain_wake_pipe(fd);
-                                    }
-                                    None => thread::sleep(timeout),
-                                }
+                Pop::Idle => {
+                    // Something is still in flight. Park on our own futex
+                    // until a producer wakes us (enqueue tail), a deadline
+                    // passes, or the re-check finds the world changed.
+                    //
+                    // Timekeeper (RFC 018): at most one parked scheduler
+                    // holds the timer deadline — the first idler to arm it
+                    // parks with a timeout, the rest park indefinitely, so a
+                    // timer expiry wakes one scheduler, not a herd. Peek and
+                    // arm under the timers mutex (the serialization that
+                    // makes the insert-side re-arm race-free).
+                    let tk_deadline = {
+                        let timers = match inner.timers.lock() {
+                            Ok(t) => t,
+                            Err(e) => {
+                                panic!("smarm: timers lock poisoned (core corrupt): {e}")
                             }
-                        }
-                        (None, Some(fd)) if io_outstanding > 0 => {
-                            crate::io::poll_wake(fd, None);
-                            crate::io::drain_wake_pipe(fd);
-                        }
-                        _ => {
-                            thread::sleep(std::time::Duration::from_micros(100));
-                        }
+                        };
+                        timers
+                            .peek_deadline()
+                            .filter(|d| inner.coord.try_arm_timer(slot_idx, *d))
+                    };
+                    // The mandatory post-publish re-check: a producer that
+                    // enqueued (or a verdict input that flipped) before it
+                    // could see our idle bit has left us the evidence.
+                    let _ = inner.coord.park(slot_idx, tk_deadline, || {
+                        !inner.run_queue.is_empty()
+                            || (inner.live_actors.load(Ordering::Acquire) == 0
+                                && inner.io_outstanding.load(Ordering::Acquire) == 0
+                                && inner.io_fd_waiters.load(Ordering::Acquire) == 0)
+                            || (inner.root_exited.load(Ordering::Acquire)
+                                && !inner.root_swept.load(Ordering::Acquire))
+                            || inner.coord.deadline_due()
+                    });
+                    if tk_deadline.is_some() {
+                        // Hand the role back BEFORE firing: pop_due can run
+                        // `Send` thunks that insert new timers, and the
+                        // insert-side re-arm check must see either no
+                        // timekeeper (skip) or a real parked one — never us,
+                        // awake and about to re-peek anyway.
+                        inner.coord.disarm_timer(slot_idx);
+                        // Woken for the deadline, for work, or to re-peek
+                        // after an earlier insert — fire whatever is due;
+                        // the next idle pass re-arms with the new minimum.
+                        fire_due_timers(inner, false);
                     }
                     continue;
                 }
@@ -1515,6 +1767,21 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
         //    by the at-most-once-enqueued invariant nothing else can have
         //    changed the state of a queued actor.
         // ----------------------------------------------------------------
+
+        // RFC 018 chain rule: we just took one runnable; if more remain and
+        // a sibling is parked, wake exactly one so the surplus runs in
+        // PARALLEL rather than serially behind us (without this the surplus
+        // is not stranded — we re-pop it after resuming — but it waits out
+        // our whole timeslice while an idle core sits available). Cheap: the
+        // queue-length check is queue-local, and `wake_one_if_idle` is a
+        // fence + one Relaxed mask load when nobody is parked. A Relaxed
+        // miss here is safe — the enqueue that created the surplus already
+        // issued its own wake (RFC 018 no-lost-wake); this only sharpens
+        // parallelism latency.
+        if !inner.run_queue.is_empty() {
+            inner.coord.wake_one_if_idle();
+        }
+
         let slot = match inner.slot_at(pid) {
             Some(s) => s,
             None => continue, // can't happen for real pids; defensive
@@ -1553,6 +1820,11 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
             crate::preempt::reset_timeslice();
         }
         PREEMPTION_ENABLED.with(|c| c.set(true));
+        // RFC 007: delay accrued while this actor was off-CPU is absorbed for
+        // free (Coz's blocked-thread rule) — fast-forward its ledger and arm
+        // the per-thread sample clock before it runs.
+        #[cfg(feature = "smarm-causal")]
+        crate::causal::on_resume(slot);
 
         crate::te!(crate::trace::Event::Resume(pid));
         unsafe { switch_to_actor() };
@@ -1579,6 +1851,11 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
             let gen = pid.generation();
             match intent {
                 YieldIntent::Yield => {
+                    // RFC 007 audit: measure the sample tail this yield drops
+                    // (near-zero for slice-expiry yields, which sample at the
+                    // same checkpoint; fat for explicit yield_now in-site).
+                    #[cfg(feature = "smarm-causal")]
+                    crate::causal::on_deschedule(slot, false);
                     // Running OR RunningNotified → Queued; a notification
                     // arriving mid-run coalesces into the re-queue.
                     crate::te!(crate::trace::Event::Yield(pid));
@@ -1587,11 +1864,24 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
                 }
                 YieldIntent::Park => {
                     if slot.word.park_return(gen) {
+                        // RFC 007 audit: an in-site park drops its sample
+                        // tail (nothing flushes it; on_resume re-arms).
+                        #[cfg(feature = "smarm-causal")]
+                        crate::causal::on_deschedule(slot, true);
+                        // RFC 007: a real park — the eventual wake forgives
+                        // delay accrued while blocked (the instant-wake
+                        // re-queue below does NOT: that actor never blocked).
+                        #[cfg(feature = "smarm-causal")]
+                        slot.set_causal_parked();
                         crate::te!(crate::trace::Event::Park(pid));
                     } else {
                         // An unpark landed in the prep-to-park window; the
                         // word is back to Queued — re-queue instead of
-                        // parking. The lost-wakeup window, closed.
+                        // parking. The lost-wakeup window, closed: this actor
+                        // never blocked, so its dropped tail counts as a
+                        // yield in the RFC 007 audit.
+                        #[cfg(feature = "smarm-causal")]
+                        crate::causal::on_deschedule(slot, false);
                         crate::te!(crate::trace::Event::UnparkFlagConsumed(pid));
                         inner.enqueue(pid);
                     }

src/supervisor.rs

@@ -1,16 +1,114 @@
-//! Supervision signals.
+//! Supervision: keep a set of actors alive.
 //!
-//! Every actor has a supervisor, which is itself just an actor with a
-//! `Receiver<Signal>`. When a child actor terminates, the scheduler sends
-//! a `Signal` on the supervisor's channel. The supervisor decides what to
-//! do — restart, escalate, ignore.
+//! A *supervisor* is an actor whose only job is to start a fixed set of child
+//! actors and react when one of them terminates — restarting it (and, depending
+//! on the strategy, some of its siblings) according to a policy, or giving up
+//! when failures arrive too fast. It is how you turn "an actor that might crash"
+//! into "a service that stays up": a crash becomes a restart instead of a hole
+//! in the process tree.
 //!
-//! For v0.1 there is no built-in restart-intensity cap. That's policy and
-//! lives in user code; library is mechanism only.
+//! The supervisor type is [`OneForOne`]. The name is historical — the restart
+//! *strategy* is selectable via [`OneForOne::strategy`], and
+//! [`Strategy::OneForOne`] is merely the default. You declare the children up
+//! front as [`ChildSpec`]s, each carrying a [`Restart`] policy, then hand the
+//! supervision loop an actor of its own with [`OneForOne::run`].
+//!
+//! ## A child that crashes and recovers
+//!
+//! ```
+//! use smarm::{run, spawn, ChildSpec, OneForOne, Restart};
+//! use std::sync::Arc;
+//! use std::sync::atomic::{AtomicUsize, Ordering};
+//! use std::time::Duration;
+//!
+//! run(|| {
+//!     // A flaky child: it panics on its first two starts, then settles.
+//!     let starts = Arc::new(AtomicUsize::new(0));
+//!     let s = starts.clone();
+//!     let child = move || {
+//!         let n = s.fetch_add(1, Ordering::SeqCst) + 1;
+//!         if n < 3 {
+//!             panic!("boom {n}");
+//!         }
+//!         // The third start returns normally.
+//!     };
+//!
+//!     // The supervisor runs on its own actor. `Transient` restarts a child
+//!     // that panics but treats a clean return as "done", so once the child
+//!     // finally succeeds the supervisor has nothing left to do and `run()`
+//!     // returns. smarm catches the child's panic and turns it into a restart;
+//!     // it never reaches the process as a real crash.
+//!     let sup = spawn(move || {
+//!         OneForOne::new()
+//!             .intensity(5, Duration::from_secs(60))
+//!             .child(ChildSpec::new(Restart::Transient, child))
+//!             .run();
+//!     });
+//!     sup.join().unwrap();
+//!
+//!     assert_eq!(starts.load(Ordering::SeqCst), 3); // one start, two restarts
+//! });
+//! ```
+//!
+//! ## Restart policies
+//!
+//! Each child carries a [`Restart`] policy that decides whether *that child*
+//! comes back when it terminates:
+//!
+//!   - [`Restart::Permanent`] restarts on any termination, normal or panic —
+//!     for a service that should never be down.
+//!   - [`Restart::Transient`] restarts only on an abnormal exit (a panic or a
+//!     cooperative stop); a clean return means "done" — for work that runs to
+//!     completion but should be retried if it crashes.
+//!   - [`Restart::Temporary`] never restarts; the death is simply noted.
+//!
+//! ## Strategies: which siblings get cycled
+//!
+//! When a restart is due, the [`Strategy`] decides which *other* children are
+//! cycled along with the one that died. The triggering child's own policy still
+//! decides whether anything restarts at all.
+//!
+//!   - [`Strategy::OneForOne`] restarts only the child that died — the default.
+//!   - [`Strategy::OneForAll`] restarts every child: the survivors are stopped,
+//!     then the whole set is restarted.
+//!   - [`Strategy::RestForOne`] restarts the dead child and every child started
+//!     after it, leaving earlier children untouched.
+//!
+//! ## Stopping a sibling is cooperative
+//!
+//! Cycling a sibling means stopping it first, and a supervisor never tears a
+//! running actor down from outside: smarm actors share a heap and rely on
+//! Drop/RAII, so unwinding a peer's stack from elsewhere would be unsound.
+//! Instead the supervisor *requests* the stop and the child unwinds at its next
+//! observation point — a `check!()`, an allocation, or a blocking call. A child
+//! wedged in a tight loop with no observation point cannot be stopped, for the
+//! same reason it cannot be preempted.
+//!
+//! ## Giving up: the restart-intensity cap
+//!
+//! A child that crashes the instant it starts would otherwise restart forever.
+//! [`OneForOne::intensity`] bounds that: at most `max` restarts within any
+//! `period`-long sliding window. One terminating child counts as a single
+//! restart event even when the strategy cycles several siblings. When the cap
+//! trips, the supervisor stops restarting, cooperatively stops any survivors in
+//! reverse start order, and `run()` returns.
+//!
+//! ## Running context
+//!
+//! [`OneForOne::run`] takes over the calling actor as the supervision loop, so
+//! a supervisor gets an actor of its own — typically
+//! `spawn(|| OneForOne::new()/* … */.run())`, all from inside
+//! [`run`](crate::run). Each child is spawned beneath the supervisor's pid, so
+//! every child termination funnels back to it as a [`Signal`].
 
 use crate::pid::Pid;
 use std::any::Any;
 
+/// A child-termination notice delivered to its supervisor.
+///
+/// Every child a supervisor starts is spawned beneath the supervisor's pid, so
+/// each child's termination funnels back to it as one of these. The variant
+/// records *how* the child went — which is what its [`Restart`] policy keys off.
 pub enum Signal {
     /// The child exited normally.
     Exit(Pid),
@@ -42,27 +140,6 @@ impl Signal {
     }
 }
 
-// ---------------------------------------------------------------------------
-// One-for-one supervisor
-//
-// A supervisor is itself an actor. It spawns each child under its own pid so
-// that every child death funnels into one mailbox (the `supervisor_channel`),
-// then loops: receive a Signal, decide per the child's `Restart` policy
-// whether to restart, and enforce a restart-intensity cap so a child that
-// crashes in a tight loop eventually gives up instead of spinning forever.
-//
-// What this does NOT do: *forcibly* terminate a running child. smarm actors
-// share a heap and rely on Drop/RAII, so tearing down a peer's stack from
-// outside is unsound. Stopping a sibling — whether for `one_for_all` /
-// `rest_for_one`, for a propagated link death, or for the ordered shutdown
-// below — is therefore *cooperative*: `request_stop` flags the child and it
-// unwinds at its next observation point. A child with no observation points
-// (a tight loop with no `check!()`, allocation, or blocking op) cannot be
-// stopped, exactly as it cannot be preempted. When the intensity cap trips,
-// this supervisor stops restarting and tears the remaining children down in
-// reverse start order before returning.
-// ---------------------------------------------------------------------------
-
 use crate::channel::channel;
 use std::collections::{HashMap, VecDeque};
 use std::sync::Arc;
@@ -115,11 +192,10 @@ pub enum Strategy {
 
 /// A supervisor over a fixed set of children.
 ///
-/// Despite the name (kept for backwards compatibility), the restart strategy
-/// is selectable via [`OneForOne::strategy`]; the default is
-/// [`Strategy::OneForOne`]. Build with `new()`, add children with `child()`,
-/// tune the cap with `intensity()`, then drive it with `run()` from inside an
-/// actor (typically `spawn(|| OneForOne::new()....run())`).
+/// Build it with [`new`](Self::new), add children with [`child`](Self::child),
+/// pick a [`strategy`](Self::strategy) and an [`intensity`](Self::intensity)
+/// cap, then drive the loop with [`run`](Self::run) on an actor of its own. See
+/// the [module docs](self) for the full picture.
 pub struct OneForOne {
     children: Vec<ChildSpec>,
     strategy: Strategy,
@@ -253,7 +329,7 @@ impl OneForOne {
                     .collect(),
             };
             // Stop survivors in reverse start order (highest child index first).
-            to_stop.sort_unstable_by(|a, b| b.1.cmp(&a.1));
+            to_stop.sort_unstable_by_key(|x| std::cmp::Reverse(x.1));
 
             // The set we will restart: the failed child plus every sibling we
             // are about to stop, restarted in start (ascending index) order.
@@ -299,7 +375,7 @@ impl OneForOne {
         // and this is a no-op; on a cap-trip or mailbox-closed break it tears
         // the remaining children down deterministically instead of leaking them.
         let mut survivors: Vec<(Pid, usize)> = by_pid.iter().map(|(p, i)| (*p, *i)).collect();
-        survivors.sort_unstable_by(|a, b| b.1.cmp(&a.1));
+        survivors.sort_unstable_by_key(|x| std::cmp::Reverse(x.1));
         let mut awaiting: Vec<Pid> = Vec::with_capacity(survivors.len());
         for (pid, _) in &survivors {
             crate::scheduler::request_stop(*pid);

src/sync_shim.rs

@@ -6,10 +6,19 @@
 //! Build the loom models with: `RUSTFLAGS="--cfg loom" cargo test --lib --release`
 
 #[cfg(loom)]
-pub(crate) use loom::sync::atomic::{AtomicU64, AtomicUsize, Ordering};
+pub(crate) use loom::sync::atomic::{fence, AtomicU64, AtomicUsize, Ordering};
 
 #[cfg(not(loom))]
-pub(crate) use std::sync::atomic::{AtomicU64, AtomicUsize, Ordering};
+pub(crate) use std::sync::atomic::{fence, AtomicU64, AtomicUsize, Ordering};
+
+// park.rs condvar-parker (loom + non-Linux builds only; the Linux non-loom
+// build parks on a futex and never touches these — gating them identically
+// keeps the default build free of unused imports).
+#[cfg(loom)]
+pub(crate) use loom::sync::{Condvar, Mutex};
+
+#[cfg(all(not(loom), not(target_os = "linux")))]
+pub(crate) use std::sync::{Condvar, Mutex};
 
 /// `UnsafeCell` with loom's `with`/`with_mut` access API; pass-through cost
 /// is zero in normal builds (`#[inline]`, newtype over std's cell).

src/timer.rs

@@ -102,6 +102,19 @@ pub struct Entry {
     seq: u64,
     pub pid: Pid,
     pub reason: Reason,
+    /// RFC 007 virtual time: the global delay ledger reading when this entry
+    /// was (re-)queued. `pop_due` shifts the effective deadline by any delay
+    /// injected since, so timers dilate together with the causally-delayed
+    /// workload instead of firing early in virtual terms.
+    #[cfg(feature = "smarm-causal")]
+    delay_stamp: u64,
+    /// RFC 007: a wall-anchored entry opts out of the virtual-time shift —
+    /// its deadline is honoured in wall time regardless of injected delay.
+    /// Used by the causal controller's own measurement/cooldown sleeps so
+    /// experiment windows keep a fixed wall length; ordinary workload timers
+    /// stay virtual (`false`).
+    #[cfg(feature = "smarm-causal")]
+    wall: bool,
 }
 
 impl PartialEq for Entry {
@@ -128,6 +141,14 @@ impl PartialOrd for Entry {
 
 #[derive(Default)]
 pub struct Timers {
+    /// RFC 018: the scheduler coordination layer. Attached once at
+    /// `RuntimeInner::new`; every insert notes its deadline (min-maintained
+    /// snapshot for the busy-path due-check + the timekeeper re-arm wake)
+    /// and every pop/clear re-anchors the snapshot to the heap minimum.
+    /// All calls happen under the timers mutex — the serialization the
+    /// coordinator's timer protocol mandates. `None` only in unit tests
+    /// that construct a bare `Timers`.
+    coord: Option<std::sync::Arc<crate::park::Coordinator>>,
     /// Reverse-wrapped so the smallest deadline is at the top.
     heap: BinaryHeap<Reverse<Entry>>,
     /// Monotonic counter for the tiebreaker `seq` field (and the `TimerId` of a
@@ -144,7 +165,18 @@ pub struct Timers {
 
 impl Timers {
     pub fn new() -> Self {
-        Self { heap: BinaryHeap::new(), next_seq: 0, armed: std::collections::HashSet::new() }
+        Self {
+            coord: None,
+            heap: BinaryHeap::new(),
+            next_seq: 0,
+            armed: std::collections::HashSet::new(),
+        }
+    }
+
+    /// Attach the scheduler coordination layer (RFC 018). Called once, at
+    /// runtime construction, before any scheduler thread exists.
+    pub(crate) fn attach_coordinator(&mut self, c: std::sync::Arc<crate::park::Coordinator>) {
+        self.coord = Some(c);
     }
 
     /// Insert a `Sleep` timer. Convenience for the common case.
@@ -152,6 +184,19 @@ impl Timers {
         self.insert(deadline, pid, Reason::Sleep { epoch });
     }
 
+    /// Insert a *wall-anchored* `Sleep` timer: fires at `deadline` in wall
+    /// time even while causal profiling (feature `smarm-causal`) is injecting
+    /// virtual delay — it never chases the delay ledger. Without the feature
+    /// this is identical to [`insert_sleep`](Self::insert_sleep).
+    ///
+    /// Intended for measurement machinery (the causal controller's window and
+    /// cooldown sleeps, TSC calibration) whose durations *define* wall time
+    /// rather than participate in the workload. Workload code should use the
+    /// ordinary virtual-anchored timers.
+    pub fn insert_sleep_wall(&mut self, deadline: Instant, pid: Pid, epoch: u32) {
+        self.push(deadline, pid, Reason::Sleep { epoch }, true);
+    }
+
     /// Arm a cancellable `send_after` timer: run `fire` at `deadline` unless
     /// [`cancel`](Self::cancel)led first. `pid` is informational only (the
     /// destination, or who armed it — useful for introspection); it is *not*
@@ -163,11 +208,24 @@ impl Timers {
         pid: Pid,
         fire: Box<dyn FnOnce() + Send>,
     ) -> TimerId {
-        let seq = self.next_seq;
-        self.next_seq = self.next_seq.wrapping_add(1);
-        self.armed.insert(seq);
-        self.heap.push(Reverse(Entry { deadline, seq, pid, reason: Reason::Send { fire } }));
-        TimerId(seq)
+        self.armed.insert(self.next_seq);
+        TimerId(self.push(deadline, pid, Reason::Send { fire }, false))
+    }
+
+    /// Arm a *wall-anchored* cancellable `send_after` timer (RFC 007): the
+    /// same contract as [`insert_send`](Self::insert_send), but the entry
+    /// opts out of the virtual-time shift and fires at its raw deadline
+    /// regardless of injected delay — the `Send`-reason sibling of
+    /// [`insert_sleep_wall`](Self::insert_sleep_wall). Without the
+    /// `smarm-causal` feature this is identical to `insert_send`.
+    pub fn insert_send_wall(
+        &mut self,
+        deadline: Instant,
+        pid: Pid,
+        fire: Box<dyn FnOnce() + Send>,
+    ) -> TimerId {
+        self.armed.insert(self.next_seq);
+        TimerId(self.push(deadline, pid, Reason::Send { fire }, true))
     }
 
     /// Cancel an armed `send_after` timer. Returns `true` if the timer was
@@ -179,11 +237,38 @@ impl Timers {
         self.armed.remove(&id.0)
     }
 
-    /// Insert an arbitrary timer entry.
+    /// Insert an arbitrary (virtual-anchored) timer entry.
     pub fn insert(&mut self, deadline: Instant, pid: Pid, reason: Reason) {
+        self.push(deadline, pid, reason, false);
+    }
+
+    /// Common insertion path. `wall` selects the RFC 007 anchor (see
+    /// [`insert_sleep_wall`](Self::insert_sleep_wall)); it is accepted — and
+    /// ignored — without the `smarm-causal` feature so callers don't fork.
+    /// Returns the entry's `seq`.
+    fn push(&mut self, deadline: Instant, pid: Pid, reason: Reason, wall: bool) -> u64 {
+        #[cfg(not(feature = "smarm-causal"))]
+        let _ = wall;
         let seq = self.next_seq;
         self.next_seq = self.next_seq.wrapping_add(1);
-        self.heap.push(Reverse(Entry { deadline, seq, pid, reason }));
+        self.heap.push(Reverse(Entry {
+            deadline,
+            seq,
+            pid,
+            reason,
+            #[cfg(feature = "smarm-causal")]
+            delay_stamp: crate::causal::global_delay_cycles(),
+            #[cfg(feature = "smarm-causal")]
+            wall,
+        }));
+        // RFC 018: publish the (possibly new-minimum) deadline to the
+        // busy-path snapshot and wake the timekeeper if it is parked
+        // toward a later one. We hold the timers mutex — the mandated
+        // serialization for both.
+        if let Some(c) = &self.coord {
+            c.note_deadline(deadline);
+        }
+        seq
     }
 
     pub fn is_empty(&self) -> bool {
@@ -196,6 +281,9 @@ impl Timers {
     pub fn clear(&mut self) {
         self.heap.clear();
         self.armed.clear();
+        if let Some(c) = &self.coord {
+            c.refresh_deadline(None);
+        }
     }
 
     /// Soonest pending deadline, or `None` if the heap is empty.
@@ -210,19 +298,67 @@ impl Timers {
     /// one is silently dropped here (its `seq` was already removed from
     /// `armed` by [`cancel`](Self::cancel)). Returning it removes it from
     /// `armed`, so a later `cancel` of a fired timer reports `false`.
+    ///
+    /// RFC 007 virtual time (feature `smarm-causal`): before an entry fires,
+    /// any global delay injected since it was (re-)queued is added to its
+    /// deadline; an entry whose *effective* deadline hasn't passed is pushed
+    /// back with the shifted deadline and a fresh stamp, so it keeps chasing
+    /// delay injected while it waits. Consequences, both benign:
+    /// [`peek_deadline`](Self::peek_deadline) may under-report (raw deadline
+    /// earlier than effective), costing at most one spurious scheduler wake
+    /// per injected chunk; and a shift never converts wall time — with zero
+    /// debt the path is byte-identical to the featureless one. Wall-anchored
+    /// entries ([`insert_sleep_wall`](Self::insert_sleep_wall)) are exempt
+    /// from the shift and always fire at their raw deadline.
     pub fn pop_due(&mut self, now: Instant) -> Vec<Entry> {
         let mut out = Vec::new();
+        #[cfg(feature = "smarm-causal")]
+        let global = crate::causal::global_delay_cycles();
         while let Some(r) = self.heap.peek() {
-            if r.0.deadline <= now {
-                let entry = self.heap.pop().unwrap().0;
-                if matches!(entry.reason, Reason::Send { .. }) && !self.armed.remove(&entry.seq) {
-                    // Cancelled before it came due: discard, do not deliver.
-                    continue;
-                }
-                out.push(entry);
-            } else {
+            if r.0.deadline > now {
                 break;
             }
+            #[allow(unused_mut)]
+            let mut entry = match self.heap.pop() {
+                Some(e) => e.0,
+                None => panic!("smarm: timer heap pop after peek returned None (core corrupt)"),
+            };
+            if matches!(entry.reason, Reason::Send { .. }) && !self.armed.contains(&entry.seq) {
+                // Cancelled before it came due: discard, do not deliver.
+                // (Checked before any shift so a cancelled entry is never
+                // re-queued just to be discarded later.)
+                continue;
+            }
+            #[cfg(feature = "smarm-causal")]
+            if !entry.wall {
+                let debt = global.saturating_sub(entry.delay_stamp);
+                if debt > 0 {
+                    let shifted = entry
+                        .deadline
+                        .checked_add(crate::causal::cycles_to_duration(debt))
+                        .unwrap_or(entry.deadline);
+                    if shifted > now {
+                        // Not due in virtual time: re-queue at the shifted
+                        // deadline, stamped, keeping `seq` (and thus `Send`
+                        // cancellation identity) intact.
+                        entry.deadline = shifted;
+                        entry.delay_stamp = global;
+                        self.heap.push(Reverse(entry));
+                        continue;
+                    }
+                }
+            }
+            if matches!(entry.reason, Reason::Send { .. }) {
+                self.armed.remove(&entry.seq);
+            }
+            out.push(entry);
+        }
+        // RFC 018: re-anchor the busy-path snapshot to the new heap minimum
+        // (still under the timers mutex). A causal-shift re-queue above went
+        // through `heap.push` directly, so this peek is the one place the
+        // snapshot is guaranteed to catch up.
+        if let Some(c) = &self.coord {
+            c.refresh_deadline(self.peek_deadline());
         }
         out
     }

src/trace.rs

@@ -101,7 +101,7 @@ mod inner {
 
     thread_local! {
         static LOCAL_STATE: std::cell::RefCell<Option<LocalState>> =
-            std::cell::RefCell::new(None);
+            const { std::cell::RefCell::new(None) };
     }
 
     // -----------------------------------------------------------------------
@@ -115,14 +115,20 @@ mod inner {
         let (tx, rx) = mpsc::channel::<Msg>();
         let start = Instant::now();
 
-        *GLOBAL.lock().unwrap() = Some(Global { sender: tx, start });
+        match GLOBAL.lock() {
+            Ok(mut g) => *g = Some(Global { sender: tx, start }),
+            Err(e) => panic!("smarm: trace lock poisoned (core corrupt): {e}"),
+        }
 
         // Drain thread: owns the Receiver, writes to disk.
         let path_for_thread = path.clone();
-        std::thread::Builder::new()
+        match std::thread::Builder::new()
             .name("smarm-trace-drain".into())
             .spawn(move || drain_thread(rx, &path_for_thread))
-            .expect("failed to spawn trace drain thread");
+        {
+            Ok(_) => {}
+            Err(e) => panic!("smarm: failed to spawn trace drain thread: {e}"),
+        }
 
         eprintln!("[smarm-trace] writing to {}", path);
     }
@@ -133,7 +139,10 @@ mod inner {
         // Drop the global sender so the drain thread's recv() returns Err
         // after the Flush sentinel, signalling clean shutdown.
         let sender = {
-            let mut g = GLOBAL.lock().unwrap();
+            let mut g = match GLOBAL.lock() {
+                Ok(g) => g,
+                Err(e) => panic!("smarm: trace lock poisoned (core corrupt): {e}"),
+            };
             g.take().map(|g| g.sender)
         };
         if let Some(tx) = sender {
@@ -162,7 +171,11 @@ mod inner {
             let mut opt = cell.borrow_mut();
             // Lazily initialise: one mutex hit per thread, ever.
             if opt.is_none() {
-                if let Some(g) = GLOBAL.lock().unwrap().as_ref() {
+                let guard = match GLOBAL.lock() {
+                    Ok(g) => g,
+                    Err(e) => panic!("smarm: trace lock poisoned (core corrupt): {e}"),
+                };
+                if let Some(g) = guard.as_ref() {
                     let tx = g.sender.clone();
                     *opt = Some(LocalState { tx, start: g.start });
                 }

tests/gen_statem.rs

@@ -1,76 +1,184 @@
 //! gen_statem behaviour tests, driven through the `gen_statem!` macro: cast/call
 //! round-trip, `enter` firing on start and on every real transition (but not on
-//! a stay), and the machine-down path when a handler panics.
+//! a stay), the machine-down path when a handler panics, and the timeout
+//! surface (state-timeout firing + auto-reset across transitions; named
+//! timeouts surviving transitions; cancellation).
 
 use smarm::gen_statem;
 use smarm::gen_statem::{CallError, Reply};
 use smarm::run;
 use std::sync::{Arc, Mutex};
+use std::time::Duration;
 
-// A two-state machine: Flip toggles, calls read counters, Boom panics.
+// ===========================================================================
+// Timeouts
+// ===========================================================================
+//
+// A small timer machine. `Idle` is quiet; `Armed` arms a state-timeout on entry
+// that — when it fires — bumps `st_fires` and returns to `Idle`. A named
+// timeout ("ping") is armed independently, survives the Idle/Armed transition,
+// and bumps `named_fires` when it fires. Counters are read back over calls.
+//
+// Durations are tiny and waits use `smarm::sleep` (parks the actor, leaves the
+// timer wheel turning). These tests run against real time, so they are a touch
+// slow; a controllable clock would tighten them.
 
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]
-enum Switch {
-    Off,
-    On,
+enum T {
+    Idle,
+    Armed,
 }
 
-struct Counts {
-    flips: u32,
-    enters: u32,
+struct TData {
+    enters: u32,      // total state entries (incl. initial)
+    st_fires: u32,    // state-timeout fires
+    named_fires: u32, // named-timeout fires
+    st_window: u64,   // ms for the Armed state-timeout (set per test intent)
 }
 
-enum Cast {
-    Flip,
+enum TCast {
+    Arm,        // Idle -> Armed
+    Disarm,     // Armed -> Idle (a transition that should auto-reset the state-timeout)
+    Ping(u64),  // arm a named "ping" timeout after the given ms
+    CancelPing, // cancel the named "ping"
 }
 
-enum Call {
-    GetFlips(Reply<u32>),
-    GetEnters(Reply<u32>),
-    Boom(Reply<u32>),
+enum TCall {
+    Enters(Reply<u32>),
+    StFires(Reply<u32>),
+    NamedFires(Reply<u32>),
+    Boom(Reply<u32>), // panics, to exercise the call-to-dead-machine path
 }
 
 gen_statem! {
-    machine: Sm { state: Switch, data: Counts };
-    event:   Ev { cast: Cast, call: Call };
+    machine: TimerSm { state: T, data: TData };
+    event:   Ev2 { cast: TCast, call: TCall, info: () };
     context(data, prev, cx);
 
     enter {
-        _ => data.enters += 1,
+        // On entering Armed, arm the state-timeout. On Idle, nothing — and the
+        // loop has already auto-reset any pending state-timeout on the way in.
+        T::Armed => { data.enters += 1; cx.state_timeout(Duration::from_millis(data.st_window)); },
+        T::Idle  => data.enters += 1,
     }
 
-    on Switch::Off => {
-        cast Cast::Flip => { data.flips += 1; Switch::On },
-    }
-    on Switch::On => {
-        cast Cast::Flip => Switch::Off,
+    on T::Idle => {
+        cast TCast::Arm    => T::Armed,
+        cast TCast::Disarm => unhandled,
+        state_timeout      => unhandled,
     }
 
-    // State-independent queries: reply, then stay via `prev`. Boom panics
-    // (`boom()` is typed as a state tag so the arm stays well-formed).
+    on T::Armed => {
+        // The state-timeout elapsed while still Armed: count it and go Idle.
+        state_timeout      => { data.st_fires += 1; T::Idle },
+        cast TCast::Disarm => T::Idle,
+        cast TCast::Arm    => unhandled,
+    }
+
+    // State-independent rows: the named-timeout (survives transitions), the
+    // arm/cancel casts, the counter reads, and the panicking call.
     on _ => {
-        call Call::GetFlips(r)  => { r.reply(data.flips); prev },
-        call Call::GetEnters(r) => { r.reply(data.enters); prev },
-        call Call::Boom(_r)     => boom(),
+        cast TCast::Ping(ms)      => { cx.timeout("ping", Duration::from_millis(ms)); prev },
+        cast TCast::CancelPing    => { cx.cancel_timeout("ping"); prev },
+        timeout "ping"            => { data.named_fires += 1; prev },
+        timeout _                 => unhandled,
+        call TCall::Enters(r)     => { r.reply(data.enters); prev },
+        call TCall::StFires(r)    => { r.reply(data.st_fires); prev },
+        call TCall::NamedFires(r) => { r.reply(data.named_fires); prev },
+        call TCall::Boom(_r)      => boom(),
     }
 }
 
-fn boom() -> Switch {
+fn boom() -> T {
     panic!("boom")
 }
-// Casts are applied in order and a later call observes the accumulated data.
+
+// A state-timeout armed on entry to Armed fires after its window, bumping the
+// counter and returning the machine to Idle on its own.
+#[test]
+fn state_timeout_fires() {
+    let got = Arc::new(Mutex::new(0u32));
+    let got2 = got.clone();
+    run(move || {
+        let m = TimerSm::start(T::Idle, TData { enters: 0, st_fires: 0, named_fires: 0, st_window: 5 });
+        m.send(Ev2::Cast(TCast::Arm)).unwrap(); // -> Armed, arms 5ms state-timeout
+        smarm::sleep(Duration::from_millis(40)); // let it fire
+        *got2.lock().unwrap() = m.call(|r| Ev2::Call(TCall::StFires(r))).unwrap();
+    });
+    assert_eq!(*got.lock().unwrap(), 1, "state-timeout fired exactly once");
+}
+
+// Leaving Armed before the window elapses auto-resets the state-timeout: it
+// must not fire afterward, even though we wait well past its original window.
+#[test]
+fn state_timeout_auto_resets_on_transition() {
+    let got = Arc::new(Mutex::new(99u32));
+    let got2 = got.clone();
+    run(move || {
+        // Long window so the explicit Disarm beats it comfortably.
+        let m = TimerSm::start(T::Idle, TData { enters: 0, st_fires: 0, named_fires: 0, st_window: 50 });
+        m.send(Ev2::Cast(TCast::Arm)).unwrap(); // -> Armed, arms 50ms state-timeout
+        m.send(Ev2::Cast(TCast::Disarm)).unwrap(); // -> Idle, auto-resets it
+        smarm::sleep(Duration::from_millis(80)); // past the original window
+        *got2.lock().unwrap() = m.call(|r| Ev2::Call(TCall::StFires(r))).unwrap();
+    });
+    assert_eq!(*got.lock().unwrap(), 0, "auto-reset cancelled the pending state-timeout");
+}
+
+// A named timeout survives a state change: armed in Idle, it still fires after
+// the machine has moved to Armed and back.
+#[test]
+fn named_timeout_survives_transition() {
+    let got = Arc::new(Mutex::new(0u32));
+    let got2 = got.clone();
+    run(move || {
+        // Armed's own state-timeout is long so it doesn't interfere.
+        let m = TimerSm::start(T::Idle, TData { enters: 0, st_fires: 0, named_fires: 0, st_window: 200 });
+        m.send(Ev2::Cast(TCast::Ping(20))).unwrap(); // arm "ping" for 20ms (in Idle)
+        m.send(Ev2::Cast(TCast::Arm)).unwrap(); // -> Armed (ping must survive this)
+        m.send(Ev2::Cast(TCast::Disarm)).unwrap(); // -> Idle (and this)
+        smarm::sleep(Duration::from_millis(60)); // let "ping" fire
+        *got2.lock().unwrap() = m.call(|r| Ev2::Call(TCall::NamedFires(r))).unwrap();
+    });
+    assert_eq!(*got.lock().unwrap(), 1, "named timeout fired across the transitions");
+}
+
+// Cancelling a named timeout before its window prevents the fire.
+#[test]
+fn named_timeout_cancel() {
+    let got = Arc::new(Mutex::new(99u32));
+    let got2 = got.clone();
+    run(move || {
+        let m = TimerSm::start(T::Idle, TData { enters: 0, st_fires: 0, named_fires: 0, st_window: 200 });
+        m.send(Ev2::Cast(TCast::Ping(30))).unwrap(); // arm "ping" for 30ms
+        m.send(Ev2::Cast(TCast::CancelPing)).unwrap(); // cancel before it fires
+        smarm::sleep(Duration::from_millis(60)); // past the original window
+        *got2.lock().unwrap() = m.call(|r| Ev2::Call(TCall::NamedFires(r))).unwrap();
+    });
+    assert_eq!(*got.lock().unwrap(), 0, "cancel prevented the named-timeout fire");
+}
+
+// ===========================================================================
+// Core behaviour (cast/call round-trip, enter semantics, panic -> Down)
+// ===========================================================================
+
+// Casts are applied in order and a later call observes the resulting state via
+// its counters: two Arm/Disarm round-trips leave the machine back in Idle.
 #[test]
 fn cast_then_call_roundtrip() {
     let got = Arc::new(Mutex::new(0u32));
     let got2 = got.clone();
     run(move || {
-        let sw = Sm::start(Switch::Off, Counts { flips: 0, enters: 0 });
-        sw.send(Ev::Cast(Cast::Flip)).unwrap(); // Off -> On (flips = 1)
-        sw.send(Ev::Cast(Cast::Flip)).unwrap(); // On  -> Off (no flip count)
-        let flips = sw.call(|r| Ev::Call(Call::GetFlips(r))).unwrap();
-        *got2.lock().unwrap() = flips;
+        // Long state-timeout window so it never fires during the test.
+        let m = TimerSm::start(T::Idle, TData { enters: 0, st_fires: 0, named_fires: 0, st_window: 10_000 });
+        m.send(Ev2::Cast(TCast::Arm)).unwrap();    // Idle  -> Armed (enter)
+        m.send(Ev2::Cast(TCast::Disarm)).unwrap(); // Armed -> Idle  (enter)
+        m.send(Ev2::Cast(TCast::Arm)).unwrap();    // Idle  -> Armed (enter)
+        m.send(Ev2::Cast(TCast::Disarm)).unwrap(); // Armed -> Idle  (enter)
+        // enters = 1 (start) + 4 transitions = 5.
+        *got2.lock().unwrap() = m.call(|r| Ev2::Call(TCall::Enters(r))).unwrap();
     });
-    assert_eq!(*got.lock().unwrap(), 1, "turned On once across the two flips");
+    assert_eq!(*got.lock().unwrap(), 5, "one enter on start, one per real transition");
 }
 
 // `enter` fires once on start and once per *real* transition; a stay (a call
@@ -80,14 +188,14 @@ fn enter_on_start_and_each_transition_but_not_stay() {
     let got = Arc::new(Mutex::new((0u32, 0u32, 0u32)));
     let got2 = got.clone();
     run(move || {
-        let sw = Sm::start(Switch::Off, Counts { flips: 0, enters: 0 }); // enter -> 1
-        let after_start = sw.call(|r| Ev::Call(Call::GetEnters(r))).unwrap();
-        // Two stays (the reads) must not bump enters.
-        let _ = sw.call(|r| Ev::Call(Call::GetFlips(r))).unwrap();
-        let still = sw.call(|r| Ev::Call(Call::GetEnters(r))).unwrap();
-        sw.send(Ev::Cast(Cast::Flip)).unwrap(); // Off -> On -> enter -> 2
-        let after_flip = sw.call(|r| Ev::Call(Call::GetEnters(r))).unwrap();
-        *got2.lock().unwrap() = (after_start, still, after_flip);
+        let m = TimerSm::start(T::Idle, TData { enters: 0, st_fires: 0, named_fires: 0, st_window: 10_000 }); // enter -> 1
+        let after_start = m.call(|r| Ev2::Call(TCall::Enters(r))).unwrap();
+        // A stay (a counter read returns `prev`) must not bump enters.
+        let _ = m.call(|r| Ev2::Call(TCall::StFires(r))).unwrap();
+        let still = m.call(|r| Ev2::Call(TCall::Enters(r))).unwrap();
+        m.send(Ev2::Cast(TCast::Arm)).unwrap(); // Idle -> Armed -> enter -> 2
+        let after_arm = m.call(|r| Ev2::Call(TCall::Enters(r))).unwrap();
+        *got2.lock().unwrap() = (after_start, still, after_arm);
     });
     assert_eq!(*got.lock().unwrap(), (1, 1, 2));
 }
@@ -100,9 +208,173 @@ fn call_to_panicking_handler_is_down() {
     let got = Arc::new(Mutex::new(None::<Result<u32, CallError>>));
     let got2 = got.clone();
     run(move || {
-        let sw = Sm::start(Switch::Off, Counts { flips: 0, enters: 0 });
-        let r = sw.call(|rep| Ev::Call(Call::Boom(rep)));
+        let m = TimerSm::start(T::Idle, TData { enters: 0, st_fires: 0, named_fires: 0, st_window: 10_000 });
+        let r = m.call(|rep| Ev2::Call(TCall::Boom(rep)));
         *got2.lock().unwrap() = Some(r);
     });
     assert_eq!(*got.lock().unwrap(), Some(Err(CallError::Down)));
 }
+
+// ===========================================================================
+// Postpone
+// ===========================================================================
+//
+// Two machines. `LatchSm` defers a `Take` *call* while `Empty` and answers it
+// from `Filled` — the Reply rides inside the postponed event onto the queue and
+// is honoured by whichever later state handles the replay. `RelaySm` defers a
+// `Mark` cast through two states so a replayed event can postpone *again*,
+// landing only once the handling state is reached.
+
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+enum L {
+    Empty,
+    Filled,
+}
+
+struct LData {
+    value: u32, // the value a Fill stored, handed back by a Take
+    takes: u32, // completed takes
+}
+
+enum LCast {
+    Fill(u32), // Empty -> Filled, storing the value
+}
+
+enum LCall {
+    Take(Reply<u32>),  // Filled: reply value & empty; Empty: postpone until filled
+    Takes(Reply<u32>), // completed-take count (stay)
+    Status(Reply<L>),  // current state tag (stay)
+}
+
+gen_statem! {
+    machine: LatchSm { state: L, data: LData };
+    event:   LEv { cast: LCast, call: LCall, info: () };
+    context(data, prev, cx);
+
+    enter { _ => {} }
+
+    on L::Empty => {
+        cast LCast::Fill(v) => { data.value = v; L::Filled },
+        // No value yet: defer the take (with its Reply) until a Fill arrives.
+        call LCall::Take(r)  => postpone,
+    }
+    on L::Filled => {
+        cast LCast::Fill(_) => unhandled, // already full
+        call LCall::Take(r)  => { r.reply(data.value); data.takes += 1; L::Empty },
+    }
+    on _ => {
+        call LCall::Takes(r)  => { r.reply(data.takes); prev },
+        call LCall::Status(r) => { r.reply(prev); prev },
+        state_timeout => unhandled,
+        timeout _     => unhandled,
+    }
+}
+
+// A `Take` issued while the latch is Empty parks the caller and is postponed
+// (Reply included). A later Fill transitions Empty -> Filled, whose replay
+// answers the deferred call — the parked caller wakes with the filled value.
+#[test]
+fn postponed_call_answered_after_transition() {
+    let got = Arc::new(Mutex::new(None::<u32>));
+    let g2 = got.clone();
+    run(move || {
+        let m = LatchSm::start(L::Empty, LData { value: 0, takes: 0 });
+
+        // Child actor issues the Take while Empty; its call parks on the reply.
+        let m2 = m.clone();
+        let taken = Arc::new(Mutex::new(None::<u32>));
+        let t2 = taken.clone();
+        smarm::scheduler::spawn(move || {
+            let v = m2.call(|r| LEv::Call(LCall::Take(r))).unwrap();
+            *t2.lock().unwrap() = Some(v);
+        });
+        smarm::sleep(Duration::from_millis(20)); // let the Take land and be deferred
+
+        // While deferred, the latch is untouched: still Empty, no take completed.
+        // (These reads are stays — they don't disturb the postponed event.)
+        assert_eq!(m.call(|r| LEv::Call(LCall::Status(r))).unwrap(), L::Empty);
+        assert_eq!(m.call(|r| LEv::Call(LCall::Takes(r))).unwrap(), 0);
+
+        m.send(LEv::Cast(LCast::Fill(42))).unwrap(); // Empty -> Filled: replays the Take
+        smarm::sleep(Duration::from_millis(20)); // let the child wake with its reply
+        *g2.lock().unwrap() = *taken.lock().unwrap();
+    });
+    assert_eq!(*got.lock().unwrap(), Some(42), "postponed call answered by the Filled state");
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+enum R {
+    S0,
+    S1,
+    S2,
+}
+
+struct RData {
+    marks: u32, // Marks handled (only S2 handles one)
+}
+
+enum RCast {
+    Go,   // S0 -> S1 -> S2 -> S0
+    Mark, // postponed in S0/S1, handled in S2
+}
+
+enum RCall {
+    Marks(Reply<u32>),
+    State(Reply<R>),
+}
+
+gen_statem! {
+    machine: RelaySm { state: R, data: RData };
+    event:   REv { cast: RCast, call: RCall, info: () };
+    context(data, prev, cx);
+
+    enter { _ => {} }
+
+    on R::S0 => {
+        cast RCast::Go   => R::S1,
+        cast RCast::Mark => postpone,
+    }
+    on R::S1 => {
+        cast RCast::Go   => R::S2,
+        cast RCast::Mark => postpone, // a replay here defers again
+    }
+    on R::S2 => {
+        cast RCast::Go   => R::S0,
+        cast RCast::Mark => { data.marks += 1; prev }, // finally handled
+    }
+    on _ => {
+        call RCall::Marks(r) => { r.reply(data.marks); prev },
+        call RCall::State(r) => { r.reply(prev); prev },
+        state_timeout => unhandled,
+        timeout _     => unhandled,
+    }
+}
+
+// A postponed event that is replayed into a state which *also* postpones it
+// re-queues, and is handled only once a state that accepts it is reached. Each
+// `Marks` call is a stay that acts as a sync barrier after the preceding casts.
+#[test]
+fn replayed_event_can_postpone_again() {
+    let got = Arc::new(Mutex::new((9u32, 9u32, 9u32, R::S0)));
+    let g2 = got.clone();
+    run(move || {
+        let m = RelaySm::start(R::S0, RData { marks: 0 });
+
+        m.send(REv::Cast(RCast::Mark)).unwrap(); // S0: postponed
+        let a = m.call(|r| REv::Call(RCall::Marks(r))).unwrap(); // deferred -> 0
+
+        m.send(REv::Cast(RCast::Go)).unwrap(); // S0 -> S1: replay Mark -> postponed again
+        let b = m.call(|r| REv::Call(RCall::Marks(r))).unwrap(); // re-deferred -> 0
+
+        m.send(REv::Cast(RCast::Go)).unwrap(); // S1 -> S2: replay Mark -> handled
+        let c = m.call(|r| REv::Call(RCall::Marks(r))).unwrap(); // -> 1
+        let s = m.call(|r| REv::Call(RCall::State(r))).unwrap(); // Mark was a stay in S2
+
+        *g2.lock().unwrap() = (a, b, c, s);
+    });
+    let (a, b, c, s) = *got.lock().unwrap();
+    assert_eq!(a, 0, "deferred while S0");
+    assert_eq!(b, 0, "re-deferred while S1");
+    assert_eq!(c, 1, "handled once S2 is reached");
+    assert_eq!(s, R::S2, "Mark handled as a stay in S2");
+}

tests/park_wake.rs

@@ -0,0 +1,69 @@
+//! RFC 018 scheduler park/wake — observable-behavior guards.
+//!
+//! These pin the two timer-latency properties the park/wake swap must
+//! preserve or introduce:
+//!
+//! - `sleep_fires_under_saturation`: due timers fire even when every
+//!   scheduler is busy (nobody parked ⇒ no timekeeper) — the busy-path
+//!   due-check, ratified design point (a). The old drain phase gave this
+//!   for free (timers drained every loop iteration); the new design must
+//!   not lose it.
+//! - `submillisecond_sleep_is_prompt`: a sub-ms sleep completes promptly.
+//!   Under the old wake pipe, `poll_wake`'s `as_millis` truncation turned
+//!   sub-ms deadlines into 0ms busy-polls (correct wall time, pathological
+//!   CPU); under park/wake the futex timespec carries full nanosecond
+//!   precision.
+
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
+use std::time::{Duration, Instant};
+
+#[test]
+fn sleep_fires_under_saturation() {
+    let rt = smarm::runtime::init(smarm::runtime::Config::exact(4));
+    rt.run(|| {
+        let stop = Arc::new(AtomicBool::new(false));
+        let mut spinners = Vec::new();
+        // 8 spinners over 4 schedulers: the run queue never empties, so no
+        // scheduler ever parks and no timekeeper exists. Only the busy-path
+        // due-check can fire the sleeper's timer before the spinners quit.
+        for _ in 0..8 {
+            let stop = stop.clone();
+            spinners.push(smarm::spawn(move || {
+                let t0 = Instant::now();
+                while !stop.load(Ordering::Relaxed) && t0.elapsed() < Duration::from_secs(5) {
+                    smarm::yield_now();
+                }
+            }));
+        }
+        let t0 = Instant::now();
+        smarm::sleep(Duration::from_millis(10));
+        let dt = t0.elapsed();
+        stop.store(true, Ordering::Relaxed);
+        for s in spinners {
+            let _ = s.join();
+        }
+        assert!(
+            dt < Duration::from_millis(500),
+            "10ms sleep took {dt:?} under scheduler saturation — busy-path \
+             timer firing is broken (timekeeper-only firing stalls under load)"
+        );
+    });
+}
+
+#[test]
+fn submillisecond_sleep_is_prompt() {
+    let rt = smarm::runtime::init(smarm::runtime::Config::exact(2));
+    rt.run(|| {
+        // Warm one iteration, then measure.
+        smarm::sleep(Duration::from_micros(500));
+        let t0 = Instant::now();
+        smarm::sleep(Duration::from_micros(500));
+        let dt = t0.elapsed();
+        assert!(dt >= Duration::from_micros(400), "woke early: {dt:?}");
+        assert!(
+            dt < Duration::from_millis(100),
+            "500µs sleep took {dt:?} — sub-ms deadline handling is broken"
+        );
+    });
+}

tests/runtime.rs

@@ -485,3 +485,35 @@ fn multi_thread_timer_only_no_pipe_contention() {
         SLEEP_MS * 2,
     );
 }
+
+// ---------------------------------------------------------------------------
+// Root panic propagation
+
+/// A panic in the root actor escapes `run()` to the caller. Anything else
+/// makes every assert inside `run` silently vacuous — found live when a
+/// failing-first test passed: the tripped assert was caught by the
+/// trampoline, recorded as `Outcome::Panic` on the root slot, and dropped
+/// unread with the initial handle.
+#[test]
+#[should_panic(expected = "root actor panic escapes")]
+fn root_panic_escapes_run() {
+    rt1().run(|| {
+        panic!("root actor panic escapes");
+    });
+}
+
+/// Teardown completes before the root panic propagates: a caller that
+/// catches it can immediately `run()` again on the same `Runtime` (the
+/// documented sequential-reuse contract).
+#[test]
+fn runtime_reusable_after_root_panic() {
+    let r = rt1();
+    let caught = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+        r.run(|| panic!("boom"));
+    }));
+    assert!(caught.is_err(), "root panic must escape run()");
+    let ran = Arc::new(AtomicBool::new(false));
+    let ran_t = ran.clone();
+    r.run(move || ran_t.store(true, Ordering::Relaxed));
+    assert!(ran.load(Ordering::Relaxed), "runtime unusable after root panic");
+}

tests/select.rs

@@ -108,8 +108,14 @@ fn loser_arm_wake_after_parked_select_stays_precise() {
             t0.elapsed() >= Duration::from_millis(40),
             "one-shot park returned early: a stale loser-arm wake landed"
         );
+        // Arm 0 is now closed (the sender actor exited after its sends) and
+        // a closed arm reports ready forever under priority order — observe
+        // the disconnect and drop it from the set, per the documented
+        // closed-arm rule.
+        assert_eq!(select(&[&rxa, &rxb]), 0);
+        assert!(rxa.try_recv().is_err(), "arm 0 must report disconnect");
         // The loser's message was never lost.
-        assert_eq!(select(&[&rxa, &rxb]), 1);
+        assert_eq!(select(&[&rxb]), 0);
         assert_eq!(rxb.try_recv().unwrap(), Some(2));
         h.join().unwrap();
     });

tests/stale_name_slot_reuse.rs

@@ -0,0 +1,169 @@
+//! Reproducer (soak20 signature 2, refcount_test.exs "watcher crash"):
+//! `by_name` stores only the slot *index*, so a name whose holder died — never
+//! unregistered, since no smarm stop path unregisters (prune is lazy) — and
+//! whose slot was then re-tenanted by an unrelated actor reads as *live-held*:
+//!
+//!   - `register` of the name fails `NameTaken { holder: <unrelated tenant> }`,
+//!     so the bridge's generated `start()` (a `let _ =`) silently no-ops and
+//!     `start_server/1` reports `:ok` for a server that never came up;
+//!   - a by-name `call` resolves the tenant's mailbox, misses on the message
+//!     `TypeId`, and fails `ServerDown` fast — and does NOT prune (only the
+//!     dead-holder and dangling-name arms prune), so the name never heals
+//!     while the tenant lives. The wedge is self-sustaining.
+//!
+//! Wild signature: 110235x fast `{:error, :server_down}` probes over the full
+//! 5 s await window after a swallowed restart (200-run width-20 soak, run 59).
+//!
+//! The test asserts the *contract*: after its holder dies, a name must be
+//! re-registrable regardless of what happened to the slot. Red pre-fix.
+
+use smarm::{
+    call, init, request_stop, whereis, CallError, Config, GenServer, GenServerBuilder,
+    GenServerName, RegisterError,
+};
+use std::sync::{Arc, Mutex};
+use std::time::Duration;
+
+const TARGET: GenServerName<Target> = GenServerName::new("stale_reuse_target");
+
+/// The named server whose death opens the window. Trivial on purpose.
+struct Target;
+
+impl GenServer for Target {
+    type Call = ();
+    type Reply = ();
+    type Cast = ();
+    type Info = ();
+    type Timer = ();
+
+    fn handle_call(&mut self, _req: ()) {}
+    fn handle_cast(&mut self, _op: ()) {}
+}
+
+/// The unrelated tenant. A *different* server type, so its mailbox holds a
+/// different `Envelope` `TypeId` — a same-typed tenant would make the by-name
+/// `call` *deliver to the wrong server* instead of failing, which is the same
+/// root hole wearing a worse hat.
+struct Filler;
+
+impl GenServer for Filler {
+    type Call = ();
+    type Reply = ();
+    type Cast = ();
+    type Info = ();
+    type Timer = ();
+
+    fn handle_call(&mut self, _req: ()) {}
+    fn handle_cast(&mut self, _op: ()) {}
+}
+
+#[derive(Debug)]
+struct Observed {
+    old_slot: (u32, u32),
+    tenant_slot: (u32, u32),
+    /// `whereis` of the dead name after re-tenanting — `Some` is the misread.
+    whereis_after_reuse: Option<(u32, u32)>,
+    /// By-name call after re-tenanting — the wild `server_down` fast-fail.
+    call_after_reuse: Result<(), CallError>,
+    /// The contract under test: re-registering the dead name.
+    restart: Result<(), RegisterError>,
+}
+
+#[test]
+fn dead_name_with_reused_slot_must_be_re_registrable() {
+    let out: Arc<Mutex<Option<Observed>>> = Arc::new(Mutex::new(None));
+    let out_w = out.clone();
+
+    // A deliberately tiny slab forces prompt slot recycling: with every filler
+    // held alive, the freed slot is the only *recycled* one, so a filler lands
+    // on it deterministically well before the slab (a loud panic) runs out.
+    init(Config::exact(2).max_actors(32)).run(move || {
+        // 1. Named server up; record its slot.
+        let target = GenServerBuilder::new(Target)
+            .named(TARGET)
+            .start()
+            .expect("name should be free at test start");
+        let old_pid = target.pid();
+
+        // 2. Kill it WITHOUT unregistering (no stop path does). Death is
+        //    confirmed via the *ref*, never the name — a by-name resolve of a
+        //    dead-but-not-yet-reused holder takes the prune arm and heals the
+        //    name, destroying the precondition.
+        request_stop(old_pid);
+        loop {
+            match target.call(()) {
+                Err(CallError::ServerDown) => break,
+                Ok(()) => smarm::sleep(Duration::from_millis(5)),
+            }
+        }
+        drop(target);
+
+        // 3. Re-tenant the slot: spawn fillers (all kept alive) until one
+        //    lands on the old index.
+        let mut fillers = Vec::new();
+        let mut tenant = None;
+        for i in 0..24 {
+            let name: &'static str = Box::leak(format!("stale_filler_{i}").into_boxed_str());
+            let f = GenServerBuilder::new(Filler)
+                .named(GenServerName::<Filler>::new(name))
+                .start()
+                .expect("filler names are fresh");
+            let fp = f.pid();
+            fillers.push(f);
+            if fp.index() == old_pid.index() {
+                tenant = Some(fp);
+                break;
+            }
+        }
+        let tenant = tenant.expect(
+            "precondition: the freed slot must be re-tenanted within the tiny slab \
+             (slots are recycled; every filler is held alive)",
+        );
+
+        // 4. Observe the poisoned state through the same paths the bridge uses.
+        let whereis_after_reuse = whereis(TARGET.as_str()).map(|p| (p.index(), p.generation()));
+        let call_after_reuse = call(TARGET, ());
+        let restart = GenServerBuilder::new(Target)
+            .named(TARGET)
+            .start()
+            .map(|_fresh_ref| ());
+
+        *out_w.lock().unwrap() = Some(Observed {
+            old_slot: (old_pid.index(), old_pid.generation()),
+            tenant_slot: (tenant.index(), tenant.generation()),
+            whereis_after_reuse,
+            call_after_reuse,
+            restart,
+        });
+
+        drop(fillers);
+    });
+
+    let o = out.lock().unwrap().take().expect("run body completed");
+    eprintln!("observed: {o:?}");
+
+    assert!(
+        o.restart.is_ok(),
+        "re-registering '{}' after its holder died failed with {:?}: the dead name \
+         reads as held by the live, unrelated tenant {:?} because by_name kept only \
+         the slot index (old slot {:?}). This is the silent-no-op start_server path \
+         of soak20 signature 2.",
+        TARGET.as_str(),
+        o.restart,
+        o.tenant_slot,
+        o.old_slot,
+    );
+
+    // The healed semantics around the re-register: the stale name reads
+    // *unbound* (never the tenant), and a by-name call fails ServerDown rather
+    // than resolving anything of the tenant's.
+    assert_eq!(
+        o.whereis_after_reuse, None,
+        "whereis of a dead name must prune and report unbound, not the slot's new tenant",
+    );
+    assert_eq!(
+        o.call_after_reuse,
+        Err(CallError::ServerDown),
+        "a by-name call to a dead name must fail ServerDown",
+    );
+}

tests/stop_with_queued_call.rs

@@ -0,0 +1,120 @@
+//! Reproducer: a *named* gen_server stopped with `request_stop` while a `call`
+//! sits **un-dequeued** in its inbox does NOT release the parked caller with
+//! `CallError::ServerDown`. The caller parks forever, contradicting the
+//! documented gen_server guarantee ("Any caller currently waiting in `call`
+//! sees `Err(ServerDown)`").
+//!
+//! Root cause (channel.rs): `Receiver::Drop` only flips `receiver_alive = false`
+//! and never drains `queue`. The queued `Envelope::Call(_, reply_tx)` therefore
+//! survives as long as the channel `Arc<Inner>` does — and for a *named* server
+//! the registry holds a `Sender` clone (lazy prune) that keeps the `Arc` alive
+//! after the server is gone. So the queued `reply_tx` is never dropped, the
+//! caller's `reply_rx` never closes, and `reply_rx.recv()` parks forever.
+//!
+//! Anonymous servers happen to dodge this: when their last `GenServerRef`
+//! drops, every `Sender` drops, the `Arc` refcount hits zero, `Inner` (and its
+//! queue) is dropped, and the queued `reply_tx` goes with it — waking the
+//! caller. The bug is specific to "a `Sender` outlives the `Receiver`", which a
+//! registry entry guarantees for every named server.
+
+use smarm::{
+    call, channel, init, request_stop, spawn, Config, GenServer, GenServerBuilder, GenServerName,
+    CallError, Receiver, RecvTimeoutError,
+};
+use std::sync::{Arc, Mutex};
+use std::time::Duration;
+
+const BLOCKER: GenServerName<Blocker> = GenServerName::new("repro_blocker");
+
+/// A server that, on its single cast, parks forever on a gate channel the test
+/// never feeds. This deterministically holds the server *inside a handler* (not
+/// at the inbox recv), so any subsequent `call` queues behind it and stays
+/// un-dequeued — exactly the state `request_stop` then has to clean up.
+struct Blocker {
+    gate: Option<Receiver<()>>,
+}
+
+impl GenServer for Blocker {
+    type Call = ();
+    type Reply = ();
+    type Cast = ();
+    type Info = ();
+    type Timer = ();
+
+    // Trivial + instant: if this ever ran for the queued call, the caller would
+    // get Ok(()) immediately. It must NOT run — the server is parked on the gate
+    // when the stop arrives.
+    fn handle_call(&mut self, _req: ()) {}
+
+    // Park forever (until cancelled). recv() on an open channel with no message
+    // parks the actor; the gate sender is held by the test and never fires.
+    fn handle_cast(&mut self, _op: ()) {
+        if let Some(gate) = self.gate.take() {
+            let _ = gate.recv();
+        }
+    }
+}
+
+#[test]
+fn named_server_request_stop_releases_queued_caller_with_server_down() {
+    // Final observation, asserted after the run.
+    //   Some(Err(ServerDown)) -> contract honored (fixed)
+    //   None                  -> caller never released; parked past the 3s
+    //                            bound (bug reproduced)
+    let outcome: Arc<Mutex<Option<Result<(), CallError>>>> = Arc::new(Mutex::new(None));
+    let outcome_w = outcome.clone();
+
+    init(Config::exact(2)).run(move || {
+        // Gate the server will park on. Held for the whole run so the server's
+        // gate.recv() parks (rather than seeing Disconnected and returning).
+        let (gate_tx, gate_rx) = channel::<()>();
+
+        // Channel the queued caller reports its result back on.
+        let (res_tx, res_rx) = channel::<Result<(), CallError>>();
+
+        // 1. Start the named server and keep its ref alive.
+        let server = GenServerBuilder::new(Blocker { gate: Some(gate_rx) })
+            .named(BLOCKER)
+            .start()
+            .expect("name should be free");
+        let spid = server.pid();
+
+        // 2. Send the cast and let the server dequeue it and park on the gate.
+        server.cast(()).expect("server is live");
+        smarm::sleep(Duration::from_millis(100));
+
+        // 3. A separate caller issues a by-name `call`. The server is parked on
+        //    the gate, so this Call envelope queues un-dequeued; the caller then
+        //    parks on its reply channel.
+        spawn(move || {
+            let r = call(BLOCKER, ());
+            let _ = res_tx.send(r);
+        });
+        smarm::sleep(Duration::from_millis(100));
+
+        // 4. Stop the server. Its loop unwinds out of the gate.recv() and drops
+        //    the inbox Receiver — at which point the queued caller is *supposed*
+        //    to be released with ServerDown.
+        request_stop(spid);
+
+        // 5. Bounded wait. A correct runtime releases the caller in well under
+        //    3s; the bug leaves it parked, so we time out.
+        let observed = match res_rx.recv_timeout(Duration::from_secs(3)) {
+            Ok(r) => Some(r),
+            Err(RecvTimeoutError::Timeout) => None,
+            Err(RecvTimeoutError::Disconnected) => None,
+        };
+        *outcome_w.lock().unwrap() = observed;
+
+        // Keep the gate sender alive until the very end.
+        drop(gate_tx);
+    });
+
+    let observed = outcome.lock().unwrap().take();
+    assert_eq!(
+        observed,
+        Some(Err(CallError::ServerDown)),
+        "queued caller was not released with ServerDown after the named server \
+         was request_stop'd (None = parked forever => bug reproduced)"
+    );
+}

tests/timer.rs

@@ -401,7 +401,66 @@ fn send_after_to_dead_typed_pid_is_silent() {
         assert_eq!(report_rx.recv().unwrap(), 1); // sink has now exited
         let _id = send_after(Duration::from_millis(15), sink, 2);
         sleep(Duration::from_millis(45)); // let it fire against the dead pid
-        // No panic, and nothing further delivered.
-        assert_eq!(report_rx.try_recv(), Ok(None));
+        // No panic; the sink is gone, so its report sender dropped with it —
+        // closed+empty is Err (documented), which also proves nothing
+        // further was delivered.
+        assert!(report_rx.try_recv().is_err(), "nothing further delivered");
+    });
+}
+
+// ---------------------------------------------------------------------------
+// Wall-anchored send_after (RFC 007 user-facing opt-out). The API exists in
+// both feature configs; featureless it is behaviourally identical to
+// `send_after` — these tests pin exactly that.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn armed_wall_send_timer_is_returned_and_fires() {
+    let mut t = Timers::new();
+    let now = Instant::now();
+    let fired = Arc::new(AtomicBool::new(false));
+    let f = fired.clone();
+    let _id = t.insert_send_wall(
+        now + Duration::from_millis(10),
+        Pid::new(0, 0),
+        Box::new(move || f.store(true, Ordering::SeqCst)),
+    );
+
+    let mut due = t.pop_due(now + Duration::from_millis(20));
+    assert_eq!(due.len(), 1, "an armed wall send timer should pop when due");
+    run_fire(due.pop().unwrap());
+    assert!(fired.load(Ordering::SeqCst), "running the thunk delivers");
+    assert!(t.is_empty());
+}
+
+use smarm::send_after_named_wall;
+
+#[test]
+fn send_after_named_wall_delivers_after_the_delay() {
+    const WPING: Name<u64> = Name::new("send_after_wall_ping");
+    run(|| {
+        let (tx, rx) = channel::<u64>();
+        register(WPING, tx).unwrap();
+        let t0 = Instant::now();
+        let _id = send_after_named_wall(Duration::from_millis(30), WPING, 99);
+        assert_eq!(rx.recv().unwrap(), 99);
+        assert!(
+            t0.elapsed() >= Duration::from_millis(25),
+            "delivered too early: {:?}",
+            t0.elapsed()
+        );
+    });
+}
+
+#[test]
+fn send_after_named_wall_cancels() {
+    const WC: Name<u64> = Name::new("send_after_wall_cancel");
+    run(|| {
+        let (tx, rx) = channel::<u64>();
+        register(WC, tx).unwrap();
+        let id = send_after_named_wall(Duration::from_millis(50), WC, 7);
+        assert!(cancel_timer(id), "cancel before fire returns true");
+        sleep(Duration::from_millis(90));
+        assert_eq!(rx.try_recv(), Ok(None), "cancelled wall timer delivered");
     });
 }