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).

scripts/bench_rq.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+# Run-queue shootout driver (ROADMAP_v0.5 phase 4).
+#
+# Rebuilds the runtime bench once per rq-* feature and runs the raw-structure
+# microbench once (it covers all structures in a single binary). Results land
+# in bench_results/ as full logs; the RQCSV lines are aggregated into
+# bench_results/summary.csv for plotting.
+#
+# Tune the sweep for the box, e.g. on the 20-core machine:
+#   SMARM_BENCH_THREADS="1 2 4 8 16 20" ./scripts/bench_rq.sh
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+OUT=bench_results
+mkdir -p "$OUT"
+: "${SMARM_BENCH_THREADS:=1 2 4}"
+export SMARM_BENCH_THREADS
+
+echo "== raw structures (one binary, all variants) =="
+cargo bench --bench rq_micro 2>&1 | tee "$OUT/micro.txt"
+
+for v in rq-mutex rq-mpmc rq-striped; do
+  echo "== runtime benches: $v =="
+  cargo bench --bench rq_runtime --no-default-features --features "$v" \
+    2>&1 | tee "$OUT/runtime-$v.txt"
+done
+
+echo "bench,kind,a,b,c,d,median_us,ops_per_s" > "$OUT/summary.csv"
+grep -h '^RQCSV,' "$OUT"/*.txt | sed 's/^RQCSV,//' >> "$OUT/summary.csv"
+echo
+echo "Summary: $OUT/summary.csv ($(($(wc -l < "$OUT/summary.csv") - 1)) rows)"