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