Preserve audio and artwork while replacing conflicting Opus gain tags with R128 comments. Verify manual, download, and album writes, handle SAF failures, and refresh playback normalization after saving.
Pure move: paths, native worker, finalization, ReplayGain, and
metadata embedding move to part files as private extensions on
DownloadQueueNotifier; queue orchestration, state, and public API
stay in the main file (7598 -> 4022 lines).