resolveSubtitleTrack matches a Jellyfin subtitle against the player's real
track list (external by external-filename, embedded by language/title) instead
of positional counting, which mis-selects when externals/embedded are reordered
or the server hides embedded subs. applyMpvSubtitleSelection is the shared entry
point (reusable for the cast backend). compareTracksForMenu mirrors web's
itemHelper.sortTracks. Drops dead getMpvSubtitleId/isSubtitleInMpv. 14 unit tests.
ceeacda7f9