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