write_tags_to_file wrote the core fields + cover but never the source IDs
(Spotify/iTunes/MusicBrainz) the import post-process embeds. Added a focused
source.embed_known_source_ids() that writes ALREADY-KNOWN ids (from db_data)
via the canonical, Picard-compatible frame writer the import uses
(_write_embedded_metadata) — no API re-fetch, frames correct by construction.
write_tags_to_file now calls it whenever db_data carries id keys.
Fed from both paths: the enhanced-library 'Write Tags' button now carries the
track's spotify/itunes/musicbrainz ids, and the Library Re-tag job stamps the
matched album/track source ids onto each track. So both now write the full tag
set, not a subset.
3ea9da1cba