docs(roadmap): split v0.6 into up-next / later; add RFC 004 summary

Up next: channel mutex migration, named registry, gen_server call timeout.
Later: RFC 004 (tunable scheduler idle policy), unbounded slab, handle_info,
IO fd hygiene, arm-port.

ROADMAP.md

@@ -104,9 +104,7 @@ precondition.
 
 ---
 
-## v0.6 — Fast follows & open tracks
-
-Priority order within the cycle; each item is its own commit.
+## v0.6 — Up next
 
 ### 1. Channel mutex migration  (first change)
 `channel::Inner<T>` holds a `std::sync::Mutex`. Phase 2 surfaced a sharp
@@ -126,37 +124,49 @@ keep it cheap/pure.
 `HashMap<String, Pid>` in `SharedState`, guarded by the shared lock. No
 architecture changes; can land immediately after #1.
 
-### 3. gen_server: handle_info + call timeout
-- `handle_info`: needs a cross-channel mailbox merge (call/cast inbox +
-  out-of-band signals). The still-unmade decision: whether to introduce a
-  unified per-actor mailbox or keep per-channel `recv_match` composition.
-- Call timeout: `call` with a deadline — monitor the server, wait
-  reply-or-Down-or-deadline, then `demonitor`+drop so a timed-out call leaks
-  no registration. Requires a per-`recv` deadline / `Signal::Timeout` primitive
-  that doesn't exist yet.
+### 3. gen_server: call timeout
+`call` with a deadline — monitor the server, wait reply-or-Down-or-deadline,
+then `demonitor`+drop so a timed-out call leaks no registration. Requires a
+per-`recv` deadline / `Signal::Timeout` primitive that doesn't exist yet.
+(`handle_info` deferred — needs cross-channel mailbox merge; see Later.)
 
-  These two can land independently; call timeout is smaller and more useful
-  first.
+---
 
-### 4. Unbounded / configurable-bounded actor count
-v0.5 ships a fixed slab with a loud assert (`Config::max_actors(n)`, default
-16 384). Revisit with a segmented slab (array of `AtomicPtr<Segment>`, doubling
-segment sizes, append-only) once the cap is actually hit or a workload demands
-it. Do not let the fixed cap calcify into an assumption.
+## Later
 
-### 5. Idle-wakeup eventcount
-Idle scheduler threads currently poll-sleep at 100 µs. A futex-based eventcount
-would reduce idle latency. Defer until benches show it matters.
+### Tunable scheduler idle policy (RFC 004)
+Today idle schedulers fall back to `thread::sleep(100µs)`, putting ~100µs
+worst-case latency on every cross-thread actor handoff. RFC 004 specifies a
+bounded spinning-worker policy (Go/Tokio model): idle schedulers spin on a
+cheap atomic for a configurable budget before parking, so a burst of work is
+picked up in cache-coherency time rather than after a kernel reschedule.
 
-### 6. IO fd hygiene on actor death
+Two new `Config` knobs: `spin_budget_cycles` (RDTSC cycles to spin; `0`
+recovers today's behaviour) and `max_spinners` (cap on concurrent spinners;
+default `N/2`). Single new atomic `n_spinning`. No change to the run-queue
+data structure — lands on the current `Mutex<VecDeque>` and is independent of
+any future queue topology work.
+
+Full spec: `rfc_004-tunable-scheduler-idle-policy.md` (artefacts.kalsbeek.dev).
+
+### Unbounded / configurable-bounded actor count
+Fixed slab with a loud assert (`Config::max_actors(n)`, default 16 384).
+Revisit with a segmented slab (array of `AtomicPtr<Segment>`, doubling segment
+sizes, append-only) once the cap is actually hit. Do not let it calcify.
+
+### gen_server: handle_info
+Needs a cross-channel mailbox merge (call/cast inbox + out-of-band signals) —
+the still-unmade decision between a unified per-actor mailbox and per-channel
+`recv_match` composition.
+
+### IO fd hygiene on actor death
 Pre-existing v0.2 TODO in `io.rs`: fds registered with epoll for a dead actor
 are not cleaned up. Audit and fix.
 
-### 7. arm-port validation & merge
+### arm-port validation & merge
 `arm-port` branch carries an AAPCS64 context-switch backend, never run on
-hardware. Steps: build + run full test suite on an aarch64 device; check
-`chained_spawn` / `yield_many` bench medians for regressions vs x86 baseline;
-merge and update README module table to reflect multi-arch support.
+hardware. Build + run full test suite on an aarch64 device; check
+`chained_spawn` / `yield_many` bench medians; merge and update README.
 
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