fix(demo): pace gait windows before buffering

Make the OpenGait-studio demo drop unpaced frames before they grow the silhouette window. Separate source-frame gap tracking from paced-frame stride tracking so runtime scheduling matches the documented demo-window-and-stride behavior.

Add regressions for paced window growth and schedule-frame stride semantics.

tests/opengait_studio/test_window.py

@@ -144,6 +144,21 @@ class TestSilhouetteWindow:
         window.push(sil, frame_idx=7, track_id=1)
         assert window.should_classify()
 
+    def test_should_classify_uses_schedule_frame_idx(self) -> None:
+        """Stride should be measured in paced/scheduled frames, not source frames."""
+        window = SilhouetteWindow(window_size=2, stride=2, gap_threshold=50)
+        sil = np.ones((64, 44), dtype=np.float32)
+
+        window.push(sil, frame_idx=0, track_id=1, schedule_frame_idx=0)
+        window.push(sil, frame_idx=10, track_id=1, schedule_frame_idx=1)
+
+        assert window.should_classify()
+
+        window.mark_classified()
+        window.push(sil, frame_idx=20, track_id=1, schedule_frame_idx=2)
+
+        assert not window.should_classify()
+
     def test_should_classify_not_ready(self) -> None:
         """should_classify should return False when window not ready."""
         window = SilhouetteWindow(window_size=5, stride=1, gap_threshold=10)