Add `episodes_per_video_file` to `episodes_factory`: episode `i` goes to
`file_index = i // episodes_per_video_file` and `from_timestamp` restarts at 0 on
each rollover. `create_videos` encodes one .mp4 per file, and
`mock_snapshot_download` lists and creates every video file rather than only
file-000. Without the option the fixtures emit the same single-file layout as
before.
Add streaming tests over that layout, on the plain and the delta path.
dongmao.zhang
e2804c9fbd