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/pipeline.py

@@ -29,6 +29,7 @@ class PersonAnalysis:
     tracked_pose: TrackedPersonPose
     pose_evidence: PoseEvidence
     state: FallState
+    evidence: Evidence
 
 
 @dataclass(frozen=True)
@@ -129,6 +130,7 @@ class FallPipeline:
                     tracked_pose=tracked,
                     pose_evidence=pose_evidence,
                     state=self._state_machine.state_of(tracked.track_id),
+                    evidence=evidence,
                 )
             )
         self._active_track_ids = current_ids