feat(bench): phase 4 — run-queue bench harness + shootout driver

- benches/rq_micro.rs: raw-structure microbench, threads x producer:consumer
  ratio sweep. Benches all three queue types in one binary (they compile in
  every build; only the runtime alias is feature-selected), so no rebuild
  dance. Queues sized to the op count so the occupancy contract is met.
- benches/rq_runtime.rs: whole-runtime benches with the selected variant:
  yield-storm (pure queue churn), ping-pong-pairs (park/unpark latency),
  spawn-storm (slab + free list + queue under churn). Scheduler-count sweep.
- scripts/bench_rq.sh: rebuilds rq_runtime per rq-* feature, runs rq_micro
  once, aggregates RQCSV lines into bench_results/summary.csv.
- All knobs via SMARM_BENCH_* env vars; house table format + machine lines.
- run_queue module is now #[doc(hidden)] pub (types + push/pop/len +
  MpmcRing::with_capacity) solely so the external bench binary can drive the
  raw structures.

docs(roadmap): phase 4 ticked (harness done; numbers from the 20-core box).
New fast-follow per review: assert the invariants we lean on — debug_assert!
on hot paths, loud assert!/panic on cold ones, at the point of reliance;
sweep existing code during the phase-5 audit, adopt as house style.

Validated end-to-end at smoke scale on the 1-core sandbox: full driver run,
24-row summary.csv across micro (3 structures x sweeps) and runtime
(3 variants x 3 benches x thread sweep).

.gitignore

@@ -1,3 +1,4 @@
 target
 Cargo.lock
 smarm_trace.json
+/bench_results/