perf(run_queue): flip default backend rq-mutex -> rq-mpmc

Evidence (history.md session 5, findings 10-11, 20-core jobrunner run):
rq-mutex collapses with thread count on queue-heavy load (yield-storm
6.9x slower than striped at 20T; attributed cause of the baseline's
multi-thread regression on yield_many/chained_spawn), while rq-mpmc is
best-or-close everywhere: best 1-thread, best slot-on ping-pong
(1237us vs mutex 6754us at 20T), -23%/-41 cyc per yield roundtrip on
real hardware (single-core switch_cost, interleaved). rq-striped
remains the churn-heavy many-core option, selectable per build.

Docs + compile_error hints in run_queue.rs updated to name rq-mpmc as
the default. slot_state.rs untouched, no loom-relevant changes; lib
(60) + scheduler/channel/supervisor/park_wake/wake_slot/preempt (41)
pass under the new default.

Cargo.toml

@@ -17,7 +17,7 @@ unwrap_used = "deny"
 expect_used = "deny"
 
 [features]
-default = ["rq-mutex"]
+default = ["rq-mpmc"]
 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

src/run_queue.rs

@@ -2,9 +2,9 @@
 //! features (no runtime dispatch — the scheduler's pop loop is the hottest
 //! code in the runtime):
 //!
-//! - `rq-mutex`   (default) — `Mutex<VecDeque>`. The control/baseline:
+//! - `rq-mutex`   — `Mutex<VecDeque>`. The control/baseline:
 //!                 strictly FIFO, trivially correct, one global lock.
-//! - `rq-mpmc`    — a single hand-rolled Vyukov bounded MPMC ring (per-cell
+//! - `rq-mpmc`    (default) — a single hand-rolled Vyukov bounded MPMC ring (per-cell
 //!                 sequence numbers). Strict FIFO, lock-free, one hot
 //!                 enqueue/dequeue cache-line pair.
 //! - `rq-striped` — M Vyukov rings with fetch-add ticket distribution.
@@ -61,20 +61,20 @@ use std::mem::MaybeUninit;
 #[cfg(not(any(feature = "rq-mutex", feature = "rq-mpmc", feature = "rq-striped")))]
 compile_error!(
     "smarm: no run queue selected. Enable exactly one of the features \
-     `rq-mutex` (default), `rq-mpmc`, `rq-striped`."
+     `rq-mpmc` (default), `rq-mutex`, `rq-striped`."
 );
 #[cfg(all(feature = "rq-mutex", feature = "rq-mpmc"))]
 compile_error!(
     "smarm: features `rq-mutex` and `rq-mpmc` are mutually exclusive \
-     (use --no-default-features to drop the default `rq-mutex`)."
+     (use --no-default-features to drop the default `rq-mpmc`)."
 );
 #[cfg(all(feature = "rq-mutex", feature = "rq-striped"))]
-compile_error!(
+compile_error!("smarm: features `rq-mutex` and `rq-striped` are mutually exclusive.");
-    "smarm: features `rq-mutex` and `rq-striped` are mutually exclusive \
-     (use --no-default-features to drop the default `rq-mutex`)."
-);
 #[cfg(all(feature = "rq-mpmc", feature = "rq-striped"))]
-compile_error!("smarm: features `rq-mpmc` and `rq-striped` are mutually exclusive.");
+compile_error!(
+    "smarm: features `rq-mpmc` and `rq-striped` are mutually exclusive \
+     (use --no-default-features to drop the default `rq-mpmc`)."
+);
 
 #[cfg(feature = "rq-mutex")]
 pub(crate) type RunQueue = MutexQueue;