Section listings ask for includeGuids=1 and the track parser reads the
<Guid id="mbid://…"/> child (only the Plex Music agent writes it; other
schemes are ignored). Stored in a new plex_cache_tracks.recording_mbid
column (additive; COALESCE on upsert) and read back by the PlexTrack
mappers. PlexCachedTrack and the qbz-source Plex constructor do not
carry it yet — the catalog projection does.
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vicrodh
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