feat: TrackRegistry — presence follows track extent

The spine of the redesign. Presence is now the extent of a track an actor owns,
[first_seen, last_seen], rather than the subset of frames in which recognition
happened to succeed. An actor recognised only at the end of a long track is
present for all of it, which is what the scene-scoped ground truth actually
records.

AR-013 — `last_seen` as an optional carries the entire liveness state: unset
means on screen, set means went off at that timestamp and still revivable,
reaped means emitted and erased. No missing-frame counter, no expired flag. It
subsumes the tracker's existing two-pool split, so there is no separate revival
path — matching a dormant track is ordinary inter-frame association.

The asymmetry is the point: interior gaps are claimed, the trailing cool-down is
not. A face lost and re-associated within the timeout never closed its track, so
the gap is presence — someone briefly occluded has not left the scene. But a
track that dies ends at its last sighting, never at the death time. That is
precisely the over-claim the retired extinction_sec keep-alive produced, where
presence ran on into the closing credits.

AR-014 — a belief swap A→B closes the track and opens a successor at the swap
frame. Not a correction: two non-twins both clearing the threshold on one face
is not realistic, whereas a track_id carried across a viewpoint change onto a
different person is. Treating it as a swap-and-continue would emit one window
blending two people; treating it as a boundary yields two that are each right.

AR-015 — two live tracks owned by one actor means at least one is wrong, since a
person cannot be in two places at once. A reverse index catches it on the update
that causes it rather than by scanning. This makes identity a third cut
detector, independent of the histogram and TransNetV2 and firing where those
failed.

AR-016 — flush() closes tracks still live at EOF. Without it a film ending
mid-shot silently drops its closing cast, which presents as a recognition miss
rather than a bookkeeping bug.

Reaping hands the dead track to the aggregator and erases it, so the registry
holds only live tracks and its size is bounded by concurrent on-screen faces
rather than growing with the film.

Locking: a frame's association pass is atomic as a unit via FrameScope, since
per-call locking would let another thread observe a half-updated frame.
owner() reads tally and verdict under one lock — separately, a track could be
both unowned and owned within a single promotion decision. A vote for an
already-reaped track is dropped and counted, because a nonzero count means the
timeout is shorter than the matcher's lag.

11 unit tests, driven directly against the registry with no network and no
fixture — the awkward cases are constructed rather than hunted for. Suite: 75
cases, 3236 assertions. Coverage 14/63 to 20/63.

Not yet wired into FaceTrackerFunc; that is AR-007/AR-008.

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

TRACES: AR-012, AR-013, AR-014, AR-015, AR-016, AR-017 | SR-002

tests/CMakeLists.txt

@@ -21,6 +21,7 @@ add_executable(sae_tests
     test_face_utils.cpp
     test_track_gallery.cpp
     test_face_tracker.cpp
+    test_track_registry.cpp
     test_audio_signature.cpp
     ${CMAKE_SOURCE_DIR}/src/backends/gemm_backend.cpp
     ${CMAKE_SOURCE_DIR}/src/gallery/gallery_store.cpp