Phase 4. core/exports/export_sources.py supplies the real I/O behind each waterfall source
and assembles resolve_fn: cache -> DB (tracks.musicbrainz_recording_id via text match) ->
file tag (UFID/musicbrainz_trackid) -> live MusicBrainz match_recording. Every source is
fail-safe (any error -> None -> fall through, export never breaks). A fresh non-cache hit is
written back to the persistent cache so the same song is free next export. Sources are
injectable; build_resolve_fn wiring (cache short-circuit + write-back) is unit-tested. 4 tests.
