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.

opengait-studio/opengait_studio/pipeline.py

@@ -148,6 +148,7 @@ class ScoliosisPipeline:
     _visualizer: OpenCVVisualizer | None
     _last_viz_payload: _VizPayload | None
     _frame_pacer: _FramePacer | None
+    _paced_frame_idx: int
     _visualizer_accepts_pose_data: bool | None
     _visualizer_signature_owner: object | None
 
@@ -209,6 +210,7 @@ class ScoliosisPipeline:
             self._visualizer = None
         self._last_viz_payload = None
         self._frame_pacer = _FramePacer(target_fps) if target_fps is not None else None
+        self._paced_frame_idx = -1
         self._visualizer_accepts_pose_data = None
         self._visualizer_signature_owner = None
 
@@ -393,8 +395,6 @@ class ScoliosisPipeline:
                 "confidence": None,
                 "pose": pose_data,
             }
-        self._window.push(silhouette, frame_idx=frame_idx, track_id=track_id)
-
         if self._frame_pacer is not None and not self._frame_pacer.should_emit(
             timestamp_ns
         ):
@@ -409,6 +409,13 @@ class ScoliosisPipeline:
                 "confidence": None,
                 "pose": pose_data,
             }
+        self._paced_frame_idx += 1
+        self._window.push(
+            silhouette,
+            frame_idx=frame_idx,
+            track_id=track_id,
+            schedule_frame_idx=self._paced_frame_idx,
+        )
         segmentation_input = self._window.buffered_silhouettes
 
         if not self._window.should_classify():