fix: a merge keeps the song link when the removed copy names the song (M498)

MergeInheritSongLink took "linked" to mean song_key <> id, but the copy
whose id became the song's key has no song_id of its own. Removing that
copy dropped the survivor out of the song. It now asks whether another
copy shares the key. The test covers both copies as the one removed, so
it no longer passes or fails on which random id sorts first.

The release-change test counted audit rows other tests left behind
(ResetDB keeps audit_log); it now counts what its own pass adds.

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

internal/library/duplicate_resolve_test.go

@@ -592,6 +592,17 @@ func TestResolveDuplicates_RecordsAReleaseChangeLidarrDidNotAnswer_Integration(t
 			if !landed {
 				lid.setErr = fmt.Errorf("%w: connection refused", lidarr.ErrUnreachable)
 			}
+			// ResetDB keeps audit_log, so other tests' release changes are
+			// still in it: count what this pass adds.
+			audited := func() (n int) {
+				t.Helper()
+				if err := f.pool.QueryRow(context.Background(),
+					`SELECT count(*) FROM audit_log WHERE action = 'lidarr_release_change'`).Scan(&n); err != nil {
+					t.Fatal(err)
+				}
+				return n
+			}
+			before := audited()
 			res, err := ResolveDuplicates(context.Background(), f.pool, nil, "", &undoingLidarr{fakeLidarr: lid, undo: !landed}, true)
 			if landed {
 				if err != nil || len(res.ReleaseChanges) != 1 {
@@ -600,13 +611,8 @@ func TestResolveDuplicates_RecordsAReleaseChangeLidarrDidNotAnswer_Integration(t
 			} else if len(res.ReleaseChanges) != 0 {
 				t.Fatalf("recorded %d changes that did not land", len(res.ReleaseChanges))
 			}
-			var audited int
-			if err := f.pool.QueryRow(context.Background(),
-				`SELECT count(*) FROM audit_log WHERE action = 'lidarr_release_change'`).Scan(&audited); err != nil {
-				t.Fatal(err)
-			}
-			if want := map[bool]int{true: 1, false: 0}[landed]; audited != want {
-				t.Errorf("audited %d release changes, want %d", audited, want)
+			if got, want := audited()-before, map[bool]int{true: 1, false: 0}[landed]; got != want {
+				t.Errorf("audited %d release changes, want %d", got, want)
 			}
 		})
 	}