PyAV segfaulted (exit 139) decoding the AV1 streams modern LeRobot
datasets use — a SIGSEGV that the per-episode try/except cannot catch,
killing the whole job when the interjections phase started.
Replace the PyAV fallback with _decode_frames_ffmpeg, which shells out
to the ffmpeg CLI: a full ffmpeg build decodes AV1, and a child-process
crash is a catchable non-zero exit rather than a segfault. Decoder chain
is now torchcodec -> ffmpeg. _decode_frames_av stays available behind
video_backend="pyav" for callers that want it.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>