MergeDuplicateGroup's loop body — repoint and copy every FK from a removed
copy onto the survivor, inherit its MBID, delete its row, tidy an emptied
album — moves into foldTrackInto, so the missing-pair pass can fold a stale
missing row into its replacement with the same mechanics. The group lock,
file removal and group bookkeeping stay in the merge. No behaviour change.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>