edge/detectors/fall.py update(): a single frame where prone was false while
state was in {FALLING, FALLEN, NO_MOTION} immediately emitted "recover" and
reset _State. One jittery frame (a momentary pose-estimation miss) could
wipe an in-progress CRITICAL no_motion incident.
Add FallConfig.recover_grace_s (default 1.0s) and _State.not_prone_since.
On the first non-prone frame while in an incident state, start the grace
timer instead of recovering immediately; only reset to STANDING once
recover_grace_s of sustained non-prone has elapsed. A re-prone frame while
in an incident state clears the pending grace timer (cancels the recover
attempt). The non-incident STANDING path is unchanged.
BEHAVIOR CHANGE: recover now requires recover_grace_s of sustained
non-prone instead of firing on a single frame.
Tests: test_recovery_clears_state (tests/test_fall.py) strengthened per
review brief to assert a single standing frame does NOT recover but
sustained standing across the grace window does. Added
test_single_noisy_nonprone_frame_does_not_cancel_no_motion covering the
life-safety scenario: one noisy non-prone frame during NO_MOTION must not
cancel the incident.
