Addresses review feedback on the failed-subtitle handling:
- Fully annotate download_tracks_in_passes and the download_track closure (MyPy strict).
- Replace list[str] skipped_subtitles with a documented SkippedSubtitle TypedDict
(id + language + title) so a client can report which subtitle of which title was
skipped; pin the shape with a contract test.
- Clear DOWNLOAD_CANCELLED in a finally so no failed track leaves it set for later code.
- Document why the subtitle pass must stay sequential (a concurrent pass would silently
drop in-flight subtitles via the cancel event).
- Warn only when a title skipped a subtitle and produced no video/audio/subtitle (was a
loose len(title.tracks) check that ignored chapters/attachments).
- Narrow the over-broad remove() except to ValueError with a debug log.
- Add tests: final-clear on the fatal path, all-subs-skipped keeps video/audio, and
duplicate-language subtitles distinguished by id.
A single failing subtitle track previously aborted the whole download. Add an opt-in
--skip-subtitle-errors flag: when set, a Subtitle failure is logged and the track dropped from
the mux while the video/audio still complete (Video/Audio failures stay fatal; default
behaviour is unchanged).
Done at the right layer to avoid the shared-event race: a failed track sets the process-global
DOWNLOAD_CANCELLED event, which makes other in-flight tracks early-return without raising — so a
skipped subtitle could otherwise silently truncate the video/audio that still got muxed. The
download is split into two passes (download_tracks_in_passes): the fatal tracks download
concurrently first, then the skippable subtitles run in a separate sequential pass once nothing
else is in flight, with the event reset before each and at the start of every title. Skipped
languages are recorded on the dl instance (skipped_subtitles) for callers to surface.
Adds tests for the cancel-event interaction (a failing subtitle no longer truncates the
video/audio), the good/bad subtitle mix, the flag-off fatal path, and the per-title reset.
