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track_max_frames_missing — the last superseded by the registry's extinction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/gallery/gallery_calibration.hpp

@@ -9,6 +9,7 @@
 #include <cstdlib>
 #include <cstring>
 #include <fstream>
+#include <functional>
 #include <iostream>
 #include <stdexcept>
 #include <string>
@@ -49,6 +50,33 @@ struct GalleryCalibration {
     }
 };
 
+/// TRACES: AR-023, AR-024 | SR-002
+///
+/// cosine → P(same person). The one probability space the pipeline reasons in.
+///
+/// Handed to every stage that has to decide whether two embeddings are the same
+/// person — track association (AR-007), evidence discounting (AR-025), identity
+/// matching — so a threshold of 0.5 means the same thing in all of them. A stage
+/// that thresholded a raw cosine instead would be using a number that means
+/// something different for every model, gallery and face size (AR-024).
+///
+/// **No prior term.** `log_prior_odds` adjusts for the gallery's base rate, which
+/// is a question about *which of N actors*; association asks whether two faces
+/// are one person, where the balanced fit is the right answer. Passing the
+/// matcher's prior here would silently bias tracking by the size of the cast.
+inline std::function<float(float)> same_person_probability(const GalleryCalibration& cal) {
+    if (!cal.valid) {
+        // Loud, because the failure mode is invisible: an untuned sigmoid still
+        // returns plausible probabilities, and every threshold downstream of it
+        // is then a guess wearing a calibrated number's clothes.
+        std::cerr << "[calibration] WARNING: no fitted calibration — association and "
+                     "evidence weighting fall back to the untuned default sigmoid "
+                     "(a=" << cal.a << ", b=" << cal.b << "). Probabilities are "
+                     "not meaningful for this embedder.\n";
+    }
+    return [cal](float similarity) { return cal.probability(similarity); };
+}
+
 // Fit a logistic sigmoid to gallery pair similarities.
 // Positive pairs: same actor, different reference images.
 // Negative pairs: different actors (all cross-actor embedding pairs).