`Le­Robot­Reader`: stream Le­Robot datasets as lazy chunk streams

LeRobotReader: stream LeRobot datasets as lazy chunk streams

rerun.experimental.LeRobotReader reads a LeRobot dataset (v2 or v3) one episode at a time:

reader = rr.experimental.LeRobotReader("path/to/dataset")
for episode in reader.episodes():
    reader.stream(episode).write_rrd(
        f"episode_{episode}.rrd",
        application_id="my_dataset",
        recording_id=f"episode_{episode}",
    )

Streaming is lazy end to end: memory is bounded by chunk size, not by episode or dataset size. Videos are cut to the episode's time window. B-frame-free video (AV1, LeRobot's default codec) streams directly; a stream that must be re-encoded — H.264 with B-frames, or a window starting mid-GOP — needs ffmpeg on the PATH.