_displacement (mean per-keypoint delta between the last two track frames,
normalized by bbox height) was duplicated verbatim in
edge/detectors/fall.py and edge/detectors/immobility.py.
Move it to edge/geometry.py as track_displacement(track) and have both
detectors call the shared helper instead of their own private copy.
Pure refactor — behavior is identical (byte-identical formula), no
config or public API changes.
Tests: tests/test_geometry.py adds direct coverage for the new public
helper (fewer-than-two-frames default, and the normalized-by-height
calculation). Existing fall/immobility tests are unchanged and stay green,
confirming behavior parity.
fall._states, immobility._still_since/_fired, zone._inside_since/_fired,
and engine._last_emit were never pruned when a track was lost (ghost
already pruned via on_lost). On a 24/7 edge device this is unbounded
growth as track IDs keep incrementing.
Add forget(track_id) to FallDetector, ImmobilityWatchdog, and
ZoneIntrusionDetector (drops that track's per-track keys/sets).
EventEngine.forget(track_id) calls each sub-detector's forget and removes
_last_emit cooldown keys whose tuple's second element is that track_id
(covers FALL/NO_MOTION/IMMOBILITY/zone/GHOST — GHOST keys are
(GHOST, track_id) after FIX 4). EventEngine.process now calls
self.forget(tr.track_id) for every track in lost_tracks after emitting,
since ghost.on_lost already consumed that track's ghost-relevant state at
the top of process().
app.py run_once already calls pool.forget(tr.track_id) for lost tracks as
part of FIX 1, completing the cleanup chain (smoother pool + detector
state + dedup cache).
Tests: tests/test_engine.py adds test_forget_prunes_state_on_lost —
fires a FALL event, then passes the track as lost, and asserts
engine._last_emit no longer has that track's keys and fall._states no
longer has the track id.
