feat(v1): per-frame fall decision diagnostics overlay

PersonAnalysis now carries the policy Evidence; view_model builds a Qt-free
per-person diagnostic line (acc/horiz/ang/rapid/cand/state, or the reject
reason) and VideoView overlays it in a corner. Read-only, no decision change;
locates where a real fall stops. 67 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

v1/view_model.py

@@ -99,6 +99,34 @@ class MonitorViewState:
     people: Tuple[PersonOverlay, ...]
     events: Tuple[EventBadge, ...]
     highest_state: FallState
+    diagnostics: Tuple[str, ...] = ()
+
+
+def person_diagnostic_line(person) -> str:
+    """Compact per-person view of the fall decision, to locate where it stops.
+
+    Shows the quality gate (``acc``), horizontal geometry (``horiz``/``ang``),
+    the rapid-drop signal (``rapid``), the policy candidate (``cand``) and the
+    state. When the pose is rejected it shows the reason instead.
+    """
+
+    track_id = person.tracked_pose.track_id
+    state = person.state.value
+    pose_evidence = person.pose_evidence
+    if pose_evidence is None or not pose_evidence.accepted:
+        reason = getattr(pose_evidence, "reason", "no_evidence")
+        return "{0} {1} acc=0 {2}".format(track_id, state, reason)
+    angle = pose_evidence.horizontal_angle_degrees
+    angle_text = "{0:.0f}deg".format(angle) if angle is not None else "-"
+    candidate = int(getattr(person.evidence, "is_fall_candidate", False))
+    return "{0} {1} acc=1 horiz={2} ang={3} rapid={4} cand={5}".format(
+        track_id,
+        state,
+        int(pose_evidence.horizontal_pose),
+        angle_text,
+        int(pose_evidence.rapid_vertical_change),
+        candidate,
+    )
 
 
 def build_monitor_view(analysis, keypoint_min_confidence: float = 0.4) -> MonitorViewState:
@@ -128,6 +156,7 @@ def build_monitor_view(analysis, keypoint_min_confidence: float = 0.4) -> Monito
         people=overlays,
         events=events,
         highest_state=_highest_state(overlays),
+        diagnostics=tuple(person_diagnostic_line(person) for person in analysis.people),
     )