fix: a slot emptied of another song's file drops that file's recording MBID, so missing-pair folding cannot fold it into the copy kept (M498)

On 2026-10-20 twelve of the 24 slots the resolver emptied were folded by MBID
into the kept copy, so their albums were never wanted back.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/duplicate_settle.go

@@ -225,7 +225,9 @@ func neverRead(ctx context.Context, q *dbq.Queries, rg string) bool {
 
 // removeWrongSlots deletes each loser's file, then marks its track missing in
 // the same transaction that closes the group. The row is kept, with its likes
-// and plays: it is the slot, and a good copy fetched again takes it over. A
+// and plays: it is the slot, and a good copy fetched again takes it over. Its
+// recording MBID is dropped, being the wrong file's, so missing-pair folding
+// does not fold the slot into the copy that was kept. A
 // file that cannot be removed stops the group; the ones already gone are still
 // marked, and the group waits for the next pass.
 func removeWrongSlots(
@@ -253,6 +255,9 @@ func removeWrongSlots(
 		if _, err := tq.MarkTracksMissing(ctx, gone); err != nil {
 			return fmt.Errorf("mark missing: %w", err)
 		}
+		if err := tq.ClearTrackMbids(ctx, gone); err != nil {
+			return fmt.Errorf("clear mbids: %w", err)
+		}
 		if err := logTrackChanges(ctx, tq, gone); err != nil {
 			return fmt.Errorf("log changes: %w", err)
 		}