fix: a file tagged with no release group joins the same-named album its folder holds

Under e45561e2 a file with no group tag took the oldest same-named album row,
which left three of Weezer's 1994 bonus tracks on the 2016 album. The lookup
now ranks same-named rows by how many tracks they hold from the file's
folder; tagReadVersion 8 re-places the existing ones.

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

internal/library/scanner_test.go

@@ -459,20 +459,29 @@ func TestScanner_SameTitledAlbumsOfTwoGroupsAreTwoAlbums_Integration(t *testing.
 		t.Fatalf("albums by group = %v, want blue with 2 tracks and green with 1", got)
 	}
 
+	// A bonus track tagged with no release group joins the album its folder
+	// holds, whichever row is older.
+	writeTestMP3(t, filepath.Join(root, "Weezer (2001)/02.mp3"), map[string]string{
+		"TIT2": "Photograph (live)", "TPE1": "Weezer", "TALB": "Weezer",
+	})
+	if _, err := scanner.Scan(ctx, nil); err != nil {
+		t.Fatalf("scan the bonus track: %v", err)
+	}
+	if got := albumsByGroup(); len(got) != 2 || got[blue] != 2 || got[green] != 2 {
+		t.Fatalf("albums by group = %v, want the ungrouped 2001 bonus track on green", 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.
+	// under an older tag-read version.
 	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 {
+	if got := albumsByGroup(); len(got) != 2 || got[blue] != 2 || got[green] != 2 {
 		t.Errorf("after the re-read, albums by group = %v, want the green track on its own album", got)
 	}
 }