One invalid byte sent a whole UTF-8 sidecar to a legacy code page, an untagged
file fell to Windows-1252 (ő/ű became õ/û) and Serbian was read as Cyrillic.
The cache key moves to sub3_ so a track the old rules mis-decoded is read again.
A Hungarian .srt beside the video (Windows-1250) was decoded as UTF-8 with
errors=replace: one episode served 1391 U+FFFD and no accented letter.
decode_subtitle now lets a BOM decide, keeps valid UTF-8, and otherwise
reads the legacy code page of the stream's language (cp1250 for Central
European, cp1251 for Cyrillic, else cp1252) — on the ffmpeg path too.
The cache has no eviction, so its key is versioned (sub2_): a track cached
broken is read again once instead of being served for good.
peter