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.
When -r included HYBRID, the dl pipeline only muxed the hybrid output and dropped every other downloaded range. DV was also treated purely as a hybrid ingredient pool, so an explicitly requested DV range never produced a standalone deliverable - only the lowest DV (the RPU ingredient) was selected.
- Select the best DV per resolution as a standalone deliverable when DV is in -r, while still using the lowest DV as the hybrid ingredient (dv_is_deliverable).
- Flag the ingredient-only DV (hybrid_base_only) so it is downloaded for the hybrid build but skipped during standalone muxing.
- Mux every requested range standalone after hybrid processing; build hybrids from deepcopied ingredients so the originals stay muxable.
- Add Tracks.merge_video_selections to de-dup the ingredient/deliverable overlap so a shared DV track is not downloaded or muxed twice.
