feat: tracker owns no state; association is frame-dependent and calibrated

Three requirements land together because they cannot be separated. The
cross-cut revival branch was the only user of cut_revive_sim, so retiring that
raw cosine forces the pool collapse, and collapsing the pool removes the only
caller of the constant. Splitting them would have produced an intermediate
commit whose only purpose was to be split.

AR-008 — FaceTrackerFunc no longer keeps its own tracks_/inactive_ maps; it
holds a shared_ptr<TrackRegistry> and operates on it directly. Two parallel
copies of track state could disagree, and every divergence would surface as a
wrong presence window with nothing to indicate it. There is now ONE candidate
pool: last_seen alone says whether IoU is meaningful. The park/revive path is
deleted outright — matching a dormant track is ordinary inter-frame
association, and continuity falls out of the embedding comparison the tracker
already did rather than being a mechanism of its own.

AR-007 — track_alpha becomes the base weight for ordinary frames only.
Association drops to embedding-only when position carries no information:
on is_cut or is_scene_boundary, because the viewpoint changed, and for a
dormant track, because time has passed since its box was last valid. The second
case matters as much as the first and had no equivalent before.

AR-024 — association cost is a calibrated probability, never a raw cosine. The
tracker takes the calibration belonging to the active embedder, the same
function object EvidenceDiscounter uses. track_max_embed_dist becomes
track_assoc_min_prob, which means the same thing for every model, gallery and
face size, where a bare cosine threshold did not.

Retired: track_max_embed_dist, cut_revive_sim, cut_inactive_max_frames, and
track_max_frames_missing — the last superseded by the registry's extinction
window. That one is worth naming: a frame count silently changed meaning with
sample_fps, so the same configuration behaved differently at 1 fps and 5 fps.
Extinction is in seconds and lives in one place.

Tests rewritten rather than deleted. The old cases asserted revival by raw
cosine; the same behaviours are now asserted through the registry — a face lost
across a cut and re-associated is the SAME track, one unbroken window, and a
face returning past the extinction window is not. Added the case AR-007 exists
for: two people swap screen positions across a cut while keeping their faces,
and identity must follow the embedding rather than the box.

Suite: 80 cases, 3250 assertions.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

TRACES: AR-007, AR-008, AR-024 | SR-002

src/nodes/identity_matcher_node.hpp

@@ -107,6 +107,14 @@ struct IdentityMatcherFunc {
         sim_engine_ = make_similarity_engine(host_gallery.data(), n_gallery_, kMaxFaces);
     }
 
+    /// TRACES: AR-023, AR-024 | SR-002
+    /// The fitted sigmoid. Exposed because the matcher is where it gets fitted
+    /// (and cached back to the gallery), but it is not the matcher's private
+    /// property: track association and evidence weighting must threshold in the
+    /// *same* probability space, or a "0.5" in one stage and a "0.5" in another
+    /// mean different things. See `same_person_probability`.
+    const GalleryCalibration& calibration() const { return cal_; }
+
     // Runtime setter — lets a persistent pipeline be reused across a threshold sweep
     // without rebuilding the (expensive, gallery-resident) matcher. The gallery,
     // calibration and GPU sim-engine stay put; only the accept threshold changes.