feat(metadata): route MP3, Ogg, and M4A edits to the native writers

EditFileMetadata now answers native_mp3/native_ogg/native_m4a and only
falls back to method=ffmpeg when a native editor rejects the file, so
every editor UI switches off the remux path without Dart changes. The
download embed path does the same for flac/mp3/opus/m4a. Side effect:
ReplayGain for MP3/Opus was silently never written (the old dispatch
answered ffmpeg and the caller ignored it) - it is now written natively.
Covers all three editors with synthetic-file tests, including foreign-tag
preservation and stco offset tracking.

.gitignore

@@ -84,6 +84,8 @@ flutter_*.log
 tool/
 .claude/settings.local.json
 .playwright-mcp/
+.rtk/
+CLAUDE.md
 
 # FVM Version Cache
 .fvm/