fix(edge): populate Event.duration_s from detector metrics (FIX 3)

edge/engine.py _mk hardcoded duration_s=0.0 for every event. The §7.3
wire contract / P1-B dashboard expects the real incident duration for
no_motion, immobility, and ghost events.

_mk now derives duration_s from the metrics dict: immobility_s (fall
no_motion, immobility watchdog) or waited_s (ghost), falling back to 0.0
for instantaneous confirms (fall, zone).

Tests: tests/test_engine.py adds test_no_motion_event_carries_duration,
driving a track to NO_MOTION and asserting the emitted event's duration_s
is > 0 and equals metrics["immobility_s"].

edge/engine.py

@@ -34,8 +34,10 @@ class EventEngine:
 
     def _mk(self, etype: EventType, now: float, track_id=None, zone_id=None,
             metrics=None, spatial=None) -> Event:
+        m = metrics or {}
+        duration_s = m.get("immobility_s") or m.get("waited_s") or 0.0
         return Event(etype, severity_for(etype), self.cfg.camera_id, track_id,
-                     zone_id, now, 0.0, metrics or {}, spatial)
+                     zone_id, now, duration_s, m, spatial)
 
     def process(self, active_tracks: list[Track], lost_tracks: list[Track],
                 now: float) -> list[Event]:

tests/test_engine.py

@@ -48,6 +48,27 @@ def test_cooldown_suppresses_duplicate():
     again = eng.process([tr], [], now=0.7)
     assert all(e.event_type != EventType.FALL for e in again)
 
+def test_no_motion_event_carries_duration():
+    eng = EventEngine(
+        FallDetector(FallConfig(prone_confirm_s=0.5, no_motion_s=1.0, motion_thr=0.05)),
+        ImmobilityWatchdog(ImmobilityConfig(immobile_s=999)),
+        GhostIncidentDetector(GhostConfig()),
+        ZoneIntrusionDetector([], ZoneConfig()),
+        EngineConfig(camera_id="cam-01", cooldown_s=30.0),
+    )
+    tr = Track(1, deque(maxlen=90))
+    _lying_track(tr, 0.0); eng.process([tr], [], now=0.0)
+    _lying_track(tr, 0.6); eng.process([tr], [], now=0.6)  # -> fall
+    _lying_track(tr, 1.0); eng.process([tr], [], now=1.0)  # still
+    _lying_track(tr, 2.0)
+    events = eng.process([tr], [], now=2.0)  # immobile >1.0s -> no_motion
+    no_motion_events = [e for e in events if e.event_type == EventType.NO_MOTION]
+    assert len(no_motion_events) == 1
+    e = no_motion_events[0]
+    assert e.duration_s > 0
+    assert e.duration_s == e.metrics["immobility_s"]
+
+
 def test_allow_cooldown_primitive():
     eng = _engine()  # EngineConfig(camera_id="cam-01", cooldown_s=30.0)
     key = ("FALL", 1)