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.
Apply resolved metadata with copyWith and retain the existing field precedence. Keep UPC, preview URLs, and source-only fields when enrichment updates other tags.
Extract the pure merge helpers and add regression tests for field preservation.
Both call sites used a bare dart:io HttpClient with no timeout, retry, or
cache; downloadCoverToFile provides all three (plus the singleflight cover
cache) and can hang-proof the embed path.
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).
zarzet