feat(introspection): I6 — regime-changed signal + per-frame analyze + honest ADR-099 D8 amendment

Three threads in this commit:

1) Per-frame attractor analysis (default analyze_every_n: 8 → 1).
   The I5 benchmark put per-frame update at 0.012 ms p99 — 83× under D4's
   1 ms budget. The cost case for the every-8th-frame default doesn't hold;
   per-frame analysis is what makes regime_changed a viable early-detection
   trigger.

2) New `regime_changed: bool` field in IntrospectionSnapshot — flips on any
   frame whose attractor regime classification differs from the previous
   frame's. Pairs with top_k_similarity (full-shape match) to give
   downstream consumers two latencies with different robustness profiles.

3) Honest amendment of ADR-099 D8 to reflect empirical reality:
   - L1 stand-in achieves 3.20× ratio (5-frame shape match vs 16-frame
     event-path floor); the 10× aspirational bar is architecturally
     unreachable at 1-D scalar feature resolution.
   - regime_changed didn't fire in the 10-frame motion window — the
     200-frame noise trajectory dominates the Lyapunov classification, and
     short perturbations don't shift the regime fast enough on a scalar
     feature.
   - Path to 10×: ADR-208 Phase 2 (Hailo NPU vec128 embeddings) — multi-dim
     partial matches discriminate from noise in 1-2 frames, not 5.
   - Side finding: midstream temporal-compare::DTW uses *discrete equality*
     cost (designed for LLM tokens), not numeric distance — swapping it in
     for f64 amplitude scoring would be strictly worse than the L1 stand-in.
     A numeric DTW is a separate concern (hand-roll or new crate).
   - Revised D8: ship behind --introspection (off by default) until multi-
     dim features land. Per-frame update budget IS met (0.041 ms p99 in this
     bench, ~24× under the 1 ms bar) — the feature is cheap enough to
     carry dark today.

cargo test -p wifi-densepose-sensing-server --no-default-features:
  introspection (lib): 8 passed, 0 failed
  introspection_latency (test): 5 passed, 0 failed (incl. new
                                 regime_change_path_latency)
clippy: clean on the introspection surface (pre-existing approx_constant
        lints in pose.rs / main.rs unchanged).

Co-Authored-By: claude-flow <ruv@ruv.net>

docs/adr/ADR-099-midstream-introspection-tap.md

@@ -118,9 +118,27 @@ Three reference signatures ship under `signatures/` in the crate as developer fi
 
 ### D8 — Measurement-first adoption — promotion bar is empirical
 
-Phase 0 spike measures the latency win against the existing `/ws/sensing` path on a recorded session. **Promotion to "ship by default" requires ≥10× p99 latency reduction on the "motion shape recognized" event class**, measured on at least one labelled recording. If the bar isn't met, the feature lives behind an `--introspection` CLI flag (default off) until it is.
+Phase 0 spike measures the latency win against the existing `/ws/sensing` path on a recorded session. **Original aspirational bar: ≥10× p99 latency reduction on the "motion shape recognized" event class**, measured on at least one labelled recording.
 
-*Consequences:* this isn't an architectural bet — the value claim is verifiable, and the feature carries its own kill switch if reality disagrees with theory.
+**Empirical baseline from `tests/introspection_latency.rs`** (I5/I6 — host-side L1 stand-in scoring + midstream-attractor regime classification on a 1-D mean-amplitude feature, 5-frame motion-ramp signature, 200 frames of noise warm-up, `analyze_every_n = 1`):
+
+| Signal | Frames to recognise | Ratio vs event-path floor (16) |
+|---|---|---|
+| `top_k_similarity[0].above_threshold` | 5 | **3.20×** |
+| `regime_changed` (10-frame motion window) | did not fire | — |
+| Per-frame `update()` p99 | **0.041 ms** (~24× under D4's 1 ms budget) | — |
+
+The 10× bar is **architecturally unreachable** at the 1-D scalar feature resolution this stand-in operates at — `signature_score`'s length-normalised L1 needs roughly the full signature length of in-shape frames to discriminate from noise (any shortcut trades false positives), and the attractor's Lyapunov classification needs more than a 10-frame perturbation to overcome a long noise trajectory. The 3.2× ratio is the structural ceiling for this feature class.
+
+**Closing the gap to 10× requires multi-dim features — specifically the `vec128` embeddings from ADR-208 Phase 2 (Hailo NPU)** — where partial matches become statistically distinguishable from noise after 1–2 frames, not 5. Until then, the adoption decision **revises the bar**:
+
+* **Ship behind `--introspection` (off by default)** until either ADR-208 P2 lands a multi-dim feature path, *or* the L1 stand-in is replaced with a numeric DTW that scores partial-prefix matches at acceptable false-positive rates.
+* The per-frame `update()` cost bar (D4: ≤1 ms p99) **is met** — the feature is cheap enough to carry dark today.
+* **Two parallel signals** in the snapshot (`top_k_similarity` for shape match, `regime_changed` for trajectory shift) cover different latency / robustness trade-offs — neither alone clears 10× on a 1-D scalar, but they cover complementary use cases. Downstream consumers pick.
+
+> **Side finding on midstream's `temporal-compare::DTW`**: its DTW uses *discrete equality* cost (0/1 between elements), not numeric distance — it's designed for LLM token sequences. On `f64` amplitude values, that scoring would be strictly worse than the L1 stand-in (every cell costs 1, no useful gradient). "Swap in midstream's DTW" — implied in earlier revisions of this ADR and proposed in I5/I6 — therefore isn't the optimization that closes D8. A *numeric* DTW would need to be hand-rolled or pulled from a different crate; tracked as a P1 follow-up alongside ADR-208 P2.
+
+*Consequences:* the kill switch is real (off-by-default CLI flag); the architectural value (continuous-state introspection surface + a per-frame regime signal + a cheap shape-match probe + a verified ≤1 ms update budget) ships, with the *latency-win* bar deferred to when multi-dim features arrive.
 
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