The testable core for the new library-wide re-tag job. Given a source album's
metadata + tracklist and the library tracks' current file tags, it:
- matches source tracks to library tracks (disc+track number, then title sim),
- computes the per-field diff (old -> new) for the dry-run finding,
- builds the minimal write_tags_to_file payload — only fields that actually
change under the chosen mode (overwrite vs fill-missing), so applying never
touches unrelated/unchanged tags.
No IO/network/DB — 10 unit tests cover matching, both modes, blank-source
fields, and the album-artist/track-count payload mapping.
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