fix(mb): rank release groups by credited artist, not title alone

The worker MB resolver picked release groups on (title_ratio, is_album)
and never scored the artist — even though resolve() knows the followed
artist. For a generic title, same-titled release groups by *different*
artists tie on title, and the is_album tiebreak (added in 06e7f91 for
Off the Wall) then actively preferred a foreign Album over the correct
release. Concretely, following Lake Street Dive's "Fun Machine" EP
resolved to an unrelated band "Bastards of Melody"'s same-titled Album,
which then propagated as the downloaded/imported artist.

Make credited-artist similarity the primary sort key, above title and
above is_album. It's a soft signal, never a hard filter, so credited-
name variations (feat., punctuation) still resolve; is_album now only
breaks ties within the same artist+title, preserving the Off the Wall
fix. Verified live: resolve("Lake Street Dive", "Fun Machine") now
returns the 6-track LSD EP.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

worker/tests/test_musicbrainz.py

@@ -4,33 +4,59 @@ from lyra_worker._musicbrainz import _best_release_group
 # path (network) is covered by test_musicbrainz_live.py.
 
 
-def _rg(title, primary_type):
-    return {"id": title.lower(), "title": title, "primary-type": primary_type}
+def _rg(title, primary_type, artist=""):
+    g = {"id": title.lower(), "title": title, "primary-type": primary_type}
+    if artist:
+        g["artist-credit"] = [{"artist": {"name": artist, "id": artist.lower()}}]
+    return g
 
 
 def test_prefers_album_over_same_titled_single():
     # Real-world "Off the Wall" bug: MusicBrainz returns the Single release-group
     # first, tied on title with the Album. We must pick the Album so a 10-track
     # download isn't tagged from the 2-track single's tracklist.
-    groups = [_rg("Off the Wall", "Single"), _rg("Off the Wall", "Album")]
-    assert _best_release_group("Off the Wall", groups)["primary-type"] == "Album"
+    groups = [
+        _rg("Off the Wall", "Single", "Michael Jackson"),
+        _rg("Off the Wall", "Album", "Michael Jackson"),
+    ]
+    assert _best_release_group("Michael Jackson", "Off the Wall", groups)["primary-type"] == "Album"
 
 
 def test_album_preference_is_order_independent():
-    groups = [_rg("Off the Wall", "Album"), _rg("Off the Wall", "Single")]
-    assert _best_release_group("Off the Wall", groups)["primary-type"] == "Album"
+    groups = [
+        _rg("Off the Wall", "Album", "Michael Jackson"),
+        _rg("Off the Wall", "Single", "Michael Jackson"),
+    ]
+    assert _best_release_group("Michael Jackson", "Off the Wall", groups)["primary-type"] == "Album"
 
 
 def test_closer_title_still_wins_over_album_type():
-    # Title similarity dominates; the album-type preference only breaks ties.
-    groups = [_rg("Continuum", "Single"), _rg("Continuum Deluxe Reissue", "Album")]
-    assert _best_release_group("Continuum", groups)["title"] == "Continuum"
+    # Title similarity dominates the album-type preference (for a single artist).
+    groups = [
+        _rg("Continuum", "Single", "John Mayer"),
+        _rg("Continuum Deluxe Reissue", "Album", "John Mayer"),
+    ]
+    assert _best_release_group("John Mayer", "Continuum", groups)["title"] == "Continuum"
 
 
 def test_below_threshold_returns_none():
-    groups = [_rg("Something Totally Different", "Album")]
-    assert _best_release_group("Off the Wall", groups) is None
+    groups = [_rg("Something Totally Different", "Album", "Michael Jackson")]
+    assert _best_release_group("Michael Jackson", "Off the Wall", groups) is None
 
 
 def test_empty_groups_returns_none():
-    assert _best_release_group("Off the Wall", []) is None
+    assert _best_release_group("Michael Jackson", "Off the Wall", []) is None
+
+
+def test_prefers_credited_artist_over_same_titled_other_artist_album():
+    # Real-world "Fun Machine" bug: Lake Street Dive's EP is what we followed, but two
+    # unrelated bands also have a release group literally titled "Fun Machine" as an Album.
+    # All three tie on title (1.0), so the old is_album tiebreak wrongly grabbed a foreign
+    # Album. The followed artist must win over title-tied albums by other artists.
+    groups = [
+        _rg("Fun Machine", "EP", "Lake Street Dive"),
+        _rg("Fun Machine", "Album", "Bastards of Melody"),
+        _rg("Fun Machine", "Album", "Teledildonics 5000"),
+    ]
+    best = _best_release_group("Lake Street Dive", "Fun Machine", groups)
+    assert best["artist-credit"][0]["artist"]["name"] == "Lake Street Dive"