docs(sim): scenario pack + LEARNING.md pointers (slice-4½ S8)

Three shipped scenarios assert distinct FOB reflex properties per spec section 5.3:
- loud-barge.toml: PRIMARY barge-in path (local VAD, 20 loud frames @ 20ms = 400ms)
- quiet-advisory.toml: SECONDARY path (sub-VAD-threshold quiet frames; in the
  slice-4-half standalone-wiring mode this exercises the harness's own latencies
  rather than real brain ASR-VAD -- the latter deferred to post-spearhead
  MockRealtimeBrain composition per spec section 8.6)
- sustained-call.toml: multi-barge / anti-fatigue check (3 loud bursts across 5 speak
  cycles; the second + third bar's kill-time should drift <= 1.5x the first's).

LEARNING.md gains 5 new slice-4-half concept pointers covering: measurement discipline
(the callers clock), post-hoc dedup of noise captures, in-process concurrency sweep,
atomic accumulators with compare_exchange_weak, and pub(crate) visibility for the
percentile_ms helper used by ConcurrencyRunner's sample-level aggregation (avoiding
the interleaved-captures-corrupt-LatencyProbe-pairing problem).

Signed-off-by: Aaron D. Lee <himself@adlee.work>

crates/rutster-sim/scenarios/loud-barge.toml

@@ -0,0 +1,27 @@
+# Scenario: loud-barge (slice-4½ spec §5.3 entry #1)
+#
+# Drives the PRIMARY barge-in path (slice-4 §5.1): the caller speaks
+# one loud burst of audio; the local VAD trips; playout dies; no brain
+# advisory needed. Asserts the wedge-#1 path: "VAD killing TTS the
+# instant the caller speaks, without the brain."
+#
+# 20 frames @ 20 ms = 400 ms of speech — comfortably past the
+# VAD_DEBOUNCE_FRAMES=3 (60 ms) debounce threshold; the VAD trips
+# deterministically.
+#
+# Threshold assertion (S7): N ∈ [1, 10, 50] concurrent SimCalls — all
+# must pass BARGE_IN_KILL_TIME_P99_MS = 80 ms at p99 (60 ms budget +
+# 20 ms observer slack).
+
+name = "loud-barge"
+
+[[steps]]
+kind = "speak_loud"
+frames = 20
+
+[[steps]]
+kind = "await_reply"
+frames = 0
+
+[[steps]]
+kind = "end"

crates/rutster-sim/scenarios/quiet-advisory.toml

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+# Scenario: quiet-advisory (slice-4½ spec §5.3 entry #2)
+#
+# Drives the SECONDARY barge-in path (slice-4 §5.2): the caller speaks
+# sub-VAD-threshold audio (zeroed PCM, energy=0, well below
+# VAD_RMS_THRESHOLD=500.0); the local VAD cannot trip; the kill must
+# come from the brain's slower ASR-VAD advisory path.
+#
+# In the slice-4½ sim harness's standalone-wiring mode, the brain side
+# is a fake-brain tokio task (no real MockRealtimeBrain WS server) —
+# this scenario exercises the advisory path via the LocalVadReflex's
+# no-op observation of quiet frames (no trip) + the awaited reply from
+# the brain task's seed reply. The kill_time + mouth_to_ear metrics
+# therefore measure the harness's own latencies, not real brain-side
+# ASR-VAD latency (the latter is deferred to the post-spearhead
+# refinement tier per spec §1.2 + §8.6 — paired with MockRealtimeBrain
+# composition + LLM-driven callers).
+#
+# Threshold assertion (S7): N=1 only (the secondary-path focus).
+
+name = "quiet-advisory"
+
+[[steps]]
+kind = "speak_quiet"
+frames = 20
+
+[[steps]]
+kind = "await_reply"
+frames = 0
+
+[[steps]]
+kind = "end"

crates/rutster-sim/scenarios/sustained-call.toml

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+# Scenario: sustained-call (slice-4½ spec §5.3 entry #3)
+#
+# Drives the multi-barge / fatigue / sustained-load check. Three loud
+# bursts + two quiet interludes + end:
+#   - loud cycle 1 → barge fires → local VAD re-arms on quiet
+#   - quiet cycle 1 → no barge, VAD re-armed for next loud
+#   - loud cycle 2 → second barge fires
+#   - quiet cycle 2 → re-arm
+#   - loud cycle 3 → third barge fires
+#
+# Threshold assertion (S7): the per-barge kill_times should drift
+# ≤ 1.5× across the three bar cycles (anti-fatigue). The second + third
+# bar's kill time should not be significantly longer than the first's
+# (no resource exhaustion, no GC pressure, no growing lock contention
+# across the slice's lifetime).
+#
+# See slice-4 §6.1 for the barge_epoch disambiguation mechanism that
+# makes "fresh re-barge" vs "late confirmation of the bar already in
+# flight" distinguishable — the Reflex::barge_epoch increments on each
+# SpeechStarted.
+
+name = "sustained-call"
+
+[[steps]]
+kind = "speak_loud"
+frames = 10
+
+[[steps]]
+kind = "speak_quiet"
+frames = 10
+
+[[steps]]
+kind = "speak_loud"
+frames = 10
+
+[[steps]]
+kind = "speak_quiet"
+frames = 10
+
+[[steps]]
+kind = "speak_loud"
+frames = 10
+
+[[steps]]
+kind = "await_reply"
+frames = 0
+
+[[steps]]
+kind = "end"