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>

v2/crates/wifi-densepose-sensing-server/src/introspection.rs

@@ -45,11 +45,13 @@ pub const DEFAULT_EMBEDDING_DIM: usize = 1;
 /// matches the snapshot carries.
 pub const DEFAULT_TOP_K: usize = 5;
 
-/// Frames since the last `analyze()` call. We don't analyse on every frame —
-/// the attractor's Lyapunov estimate is ~9 ms for a 1 k-point window per
-/// midstream's bench, which is fine at 30 Hz but wastes CPU at higher rates.
-/// One analysis every N frames stays well under the per-frame budget.
-pub const DEFAULT_ANALYZE_EVERY_N_FRAMES: u32 = 8;
+/// Frames since the last `analyze()` call. Per-frame analyse is cheap (the
+/// I5 benchmark put attractor + L1-scoring update p99 at 0.012 ms on a
+/// desktop runner, ~83× under the 1 ms D4 budget — even on a Pi 5 we have
+/// orders of magnitude of headroom), and per-frame analyse is what makes
+/// the `regime_changed` snapshot signal viable as an early-detection
+/// trigger. Default to **every frame** unless deployment tunes it down.
+pub const DEFAULT_ANALYZE_EVERY_N_FRAMES: u32 = 1;
 
 /// One labelled segment of derived feature vectors used as a DTW pattern.
 /// Schema (per ADR-099 D7) — JSON-loaded from `signatures/*.json` at startup.
@@ -153,6 +155,12 @@ pub struct IntrospectionSnapshot {
     /// Analyzer confidence in `[0, 1]`. `0.0` until the analyzer has enough
     /// data; tracks midstream's `AttractorInfo::confidence`.
     pub attractor_confidence: f64,
+    /// `true` when this frame's regime classification differs from the
+    /// previous frame's — an **early-detection signal** that doesn't require
+    /// a full signature length of frames to fire (ADR-099 D8: a parallel
+    /// fast path to the shape-match latency, useful for "something changed,
+    /// look closer" semantics on dashboards / downstream consumers).
+    pub regime_changed: bool,
     /// Top-k DTW matches against the loaded signature library. Empty when the
     /// library is empty or no signatures rose above the score floor.
     pub top_k_similarity: Vec<SimilarityMatch>,
@@ -227,6 +235,7 @@ impl IntrospectionState {
                 lyapunov_exponent: None,
                 attractor_dim: cfg.embedding_dim,
                 attractor_confidence: 0.0,
+                regime_changed: false,
                 top_k_similarity: Vec::new(),
             },
         }
@@ -263,6 +272,7 @@ impl IntrospectionState {
         // Run the (relatively expensive) analyze step every Nth frame; in
         // between, keep the previous regime/Lyapunov in the snapshot — they're
         // smooth signals, not edge-sensitive.
+        let prev_regime = self.last_snapshot.regime;
         self.frames_since_analyze = self.frames_since_analyze.saturating_add(1);
         if self.frames_since_analyze >= self.analyze_every_n {
             self.frames_since_analyze = 0;
@@ -278,6 +288,13 @@ impl IntrospectionState {
                 Err(other) => return Err(other),
             }
         }
+        // ADR-099 D8: early-detection signal — `regime_changed` flips on any
+        // frame whose classification differs from the previous frame's. Pairs
+        // with `top_k_similarity` (which needs the full shape) to give
+        // downstream consumers two latencies to choose from per use case.
+        // Don't count Unknown→Unknown as a change; do count Unknown→<any> as
+        // a change (the warm-up moment is itself informative).
+        self.last_snapshot.regime_changed = prev_regime != self.last_snapshot.regime;
 
         // DTW scoring runs every frame; cheap when the library is small (and
         // empty when it's empty). See `score_signatures` for the metric.