EditMP3Fields parses the existing ID3v2.2/2.3/2.4 tag into raw frames,
replaces only the frames the edit controls, and re-serializes as ID3v2.4
with the audio bytes streamed through untouched (temp+fsync+rename).
Unlike the ffmpeg remux this preserves foreign frames - POPM ratings,
SYLT synced lyrics, chapters, and other taggers' TXXX entries.
