fix: same-titled albums of two release groups are two albums, and a missing track with no disc is on disc 1

The scanner found an album by artist and title alone, so Weezer's six
self-titled albums (1994–2026) were one 118-track row and their merges,
wants and upgrades counted them as one. A same-named row whose release group
is known and differs from the file's is now another album; tagReadVersion 7
re-reads the library so existing tracks move to their own rows.

WantHoldings compared discs with IS NOT DISTINCT FROM, so a missing track 6
with no disc was never covered by a present disc 1 track 6. Air's Love 2
stayed wanted and was downloaded again for nothing.

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

internal/library/scanner_test.go

@@ -385,3 +385,94 @@ func TestScanner_ReleaseGroupMBID_Integration(t *testing.T) {
 		t.Errorf("album A release group changed to %v", rg)
 	}
 }
+
+// Two albums of one name by one artist, in different release groups, are two
+// albums: Weezer's six self-titled albums shared one row until 2026-10-20. A
+// track of the same group joins its album, and a track filed under the other
+// album's row by the old lookup moves to its own on the tagReadVersion re-read.
+func TestScanner_SameTitledAlbumsOfTwoGroupsAreTwoAlbums_Integration(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping scanner integration in -short mode")
+	}
+	dsn := os.Getenv("MINSTREL_TEST_DATABASE_URL")
+	if dsn == "" {
+		t.Skip("MINSTREL_TEST_DATABASE_URL not set")
+	}
+	ctx := context.Background()
+	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
+	if err := db.Migrate(dsn, logger); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+	pool, err := pgxpool.New(ctx, dsn)
+	if err != nil {
+		t.Fatalf("pool: %v", err)
+	}
+	t.Cleanup(pool.Close)
+	if _, err := pool.Exec(ctx, "TRUNCATE tracks, albums, artists RESTART IDENTITY CASCADE"); err != nil {
+		t.Fatalf("truncate: %v", err)
+	}
+
+	const (
+		blue  = "aaaaaaaa-1994-2222-3333-444444444444"
+		green = "bbbbbbbb-2001-2222-3333-444444444444"
+	)
+	root := t.TempDir()
+	tagged := func(rel, title, group string) string {
+		p := filepath.Join(root, rel)
+		writeTestMP3(t, p, map[string]string{
+			"TIT2": title, "TPE1": "Weezer", "TALB": "Weezer",
+			"TXXX": "MusicBrainz Release Group Id\x00" + group,
+		})
+		return p
+	}
+	tagged("Weezer (1994)/01.mp3", "My Name Is Jonas", blue)
+	tagged("Weezer (1994)/02.mp3", "No One Else", blue)
+	greenPath := tagged("Weezer (2001)/01.mp3", "Don't Let Go", green)
+
+	scanner := New(pool, logger, []string{root}, nil)
+	if _, err := scanner.Scan(ctx, nil); err != nil {
+		t.Fatalf("scan: %v", err)
+	}
+	albumsByGroup := func() map[string]int {
+		t.Helper()
+		rows, err := pool.Query(ctx, `SELECT a.release_group_mbid, count(t.id) FROM albums a
+			JOIN tracks t ON t.album_id = a.id WHERE a.title = 'Weezer' GROUP BY a.id, a.release_group_mbid`)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer rows.Close()
+		out := map[string]int{}
+		for rows.Next() {
+			var g *string
+			var n int
+			if err := rows.Scan(&g, &n); err != nil {
+				t.Fatal(err)
+			}
+			if g == nil {
+				t.Fatal("an album with no release group")
+			}
+			out[*g] = n
+		}
+		return out
+	}
+	if got := albumsByGroup(); len(got) != 2 || got[blue] != 2 || got[green] != 1 {
+		t.Fatalf("albums by group = %v, want blue with 2 tracks and green with 1", got)
+	}
+
+	// The green track as the old lookup left it: on the blue album's row,
+	// under an older tag-read version, its own album row gone.
+	if _, err := pool.Exec(ctx, `
+		WITH b AS (SELECT id FROM albums WHERE release_group_mbid = $1)
+		UPDATE tracks SET album_id = (SELECT id FROM b), tag_read_version = 6 WHERE file_path = $2`, blue, greenPath); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := pool.Exec(ctx, `DELETE FROM albums WHERE release_group_mbid = $1`, green); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := scanner.Scan(ctx, nil); err != nil {
+		t.Fatalf("rescan: %v", err)
+	}
+	if got := albumsByGroup(); len(got) != 2 || got[blue] != 2 || got[green] != 1 {
+		t.Errorf("after the re-read, albums by group = %v, want the green track on its own album", got)
+	}
+}