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"].
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