feat(v1): latest-frame-only reader thread for low-latency RTSP

Adds _FrameGrabber which decodes in a background thread and keeps only the
newest frame; VideoSource drop_stale (STREAM only) returns the latest frame
so a slow consumer never builds a backlog. app enables it for streams.
REPLAY is unchanged. 82 tests pass.

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

v1/app.py

@@ -38,7 +38,9 @@ class FrameWorker(QtCore.QThread):
         # OpenCV reads this env var when it opens the stream, so set it first.
         os.environ["OPENCV_FFMPEG_CAPTURE_OPTIONS"] = self._config.ffmpeg_capture_options
         mode = SourceMode.STREAM if self._config.source_mode == "stream" else SourceMode.REPLAY
-        source = VideoSource(self._config.source_url, mode=mode)
+        source = VideoSource(
+            self._config.source_url, mode=mode, drop_stale=mode is SourceMode.STREAM
+        )
         pipeline = FallPipeline.from_config(self._config, self._pose_adapter)
         try:
             while not self._stop: