Remove the never-called checkAvailability chain end to end (Dart bridge +
provider, Android/iOS MethodChannel handlers, Go exports) and stop restoring
its persistent lookup cache on every cold start; the legacy prefs key is
cleared on next cache reset.
Delete dead Go: romaji.go (test-only), the no-op pre-warm chain
(PreWarmTrackCache/PreWarmCache/…), retired Download stubs, and a batch of
exports with no Dart/Kotlin/Swift/Go caller. Kept GetTrackCacheSize/
ClearTrackIDCache (Settings cache screen), SetAllowedDownloadDirs and
Clear/GetItemProgress (live test seams).
Dedup: single updateFlacVorbis + replaceFlacPictures helper behind the six
FLAC tag writers; merge byte-identical extractCommentFrame/extractLyricsFrame
into extractLangTextFrame.
go build/vet + 237 go tests, flutter analyze + tests green.
zarzet