fix(edge): require sustained non-prone before FallDetector recover (FIX 2)

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.

edge/detectors/fall.py

@@ -17,6 +17,7 @@ class FallConfig:
     motion_thr: float = 0.02        # 정규화 관절변위 임계
     min_conf: float = 0.5
     min_visible: int = 10
+    recover_grace_s: float = 1.0    # 사고 상태에서 회복 판정까지 요구되는 지속 비-prone 시간
 
 
 @dataclass
@@ -27,6 +28,7 @@ class _State:
     still_since: float | None = None
     fall_emitted: bool = False
     no_motion_emitted: bool = False
+    not_prone_since: float | None = None
 
 
 class FallDetector:
@@ -76,12 +78,20 @@ class FallDetector:
 
         if not prone:
             if st.name in ("FALLING", "FALLEN", "NO_MOTION"):
+                # 단일 튐(noisy) 프레임만으로 CRITICAL 사고를 즉시 취소하지 않도록
+                # recover_grace_s만큼 지속된 비-prone을 요구한다.
+                if st.not_prone_since is None:
+                    st.not_prone_since = now
+                    return None, {}
+                if now - st.not_prone_since >= self.cfg.recover_grace_s:
                     self._states[track.track_id] = _State(baseline_height=bbox.height)
                     return "recover", {}
+                return None, {}
             st.prone_since = None
             return None, {}
 
-        # prone 상태
+        # prone 상태 (재-prone 프레임은 대기 중인 recover 유예를 취소)
+        st.not_prone_since = None
         if st.prone_since is None:
             st.prone_since = now
             st.name = "FALLING"

tests/test_fall.py

@@ -61,14 +61,38 @@ def test_no_motion_after_fall_escalates():
     assert sig == "no_motion"
 
 def test_recovery_clears_state():
-    det = FallDetector(FallConfig(prone_confirm_s=0.5))
+    # Recovery requires SUSTAINED non-prone for recover_grace_s, not a single frame.
+    det = FallDetector(FallConfig(prone_confirm_s=0.5, recover_grace_s=1.0))
     tr = _track()
     box = BBox(90, 290, 210, 320)
     _feed(det, tr, _lying_kps(), box, t=0.0)
     _feed(det, tr, _lying_kps(), box, t=0.6)  # fall
-    sig, _ = _feed(det, tr, _standing_kps(), BBox(40, 60, 60, 210), t=1.0)
+    standing_box = BBox(40, 60, 60, 210)
+    # a single standing frame right after a fall must NOT recover yet
+    sig, _ = _feed(det, tr, _standing_kps(), standing_box, t=1.0)
+    assert sig is None
+    # sustained standing through the grace window -> recover fires
+    sig, _ = _feed(det, tr, _standing_kps(), standing_box, t=2.5)
     assert sig == "recover"
 
+
+def test_single_noisy_nonprone_frame_does_not_cancel_no_motion():
+    det = FallDetector(FallConfig(prone_confirm_s=0.5, no_motion_s=1.0, motion_thr=0.05,
+                                   recover_grace_s=1.0))
+    tr = _track()
+    box = BBox(90, 290, 210, 320)
+    _feed(det, tr, _lying_kps(), box, t=0.0)
+    _feed(det, tr, _lying_kps(), box, t=0.6)  # -> fall
+    _feed(det, tr, _lying_kps(), box, t=1.0)  # still
+    sig, _ = _feed(det, tr, _lying_kps(), box, t=2.0)  # immobile >1.0s
+    assert sig == "no_motion"
+    # a single noisy non-prone frame within the grace window must NOT cancel
+    # the in-progress CRITICAL no_motion incident.
+    sig, _ = _feed(det, tr, _standing_kps(), BBox(40, 60, 60, 210), t=2.3)
+    assert sig is None
+    st = det._states[tr.track_id]
+    assert st.name == "NO_MOTION"
+
 def test_low_confidence_is_ignored():
     det = FallDetector(FallConfig())
     tr = _track()