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/gui.py

@@ -123,8 +123,25 @@ class VideoView(QtWidgets.QWidget):
         painter.drawImage(target, self._image)
         if self._view is not None:
             self._draw_overlays(painter, target, self._image)
+            self._draw_diagnostics(painter)
         painter.end()
 
+    def _draw_diagnostics(self, painter: QtGui.QPainter) -> None:
+        lines = list(self._view.diagnostics)
+        if not lines:
+            return
+        painter.setFont(QtGui.QFont("Consolas", 9))
+        metrics = painter.fontMetrics()
+        line_h = metrics.height()
+        box_w = max(metrics.width(line) for line in lines) + 12
+        box_h = line_h * len(lines) + 8
+        painter.fillRect(QtCore.QRect(6, 6, box_w, box_h), QtGui.QColor(0, 0, 0, 160))
+        painter.setPen(QtGui.QColor("#9BE9A8"))
+        y = 6 + line_h
+        for line in lines:
+            painter.drawText(12, y, line)
+            y += line_h
+
     def _fitted_rect(self, image: QtGui.QImage) -> QtCore.QRect:
         widget = self.rect()
         scale = min(widget.width() / image.width(), widget.height() / image.height())