fix(mb): resolve same-named albums to the Album, not the Single

MusicBrainz search returns release-groups tied on title similarity in an
arbitrary order; `max(groups, key=_ratio)` kept the first, so Michael
Jackson's "Off the Wall" resolved to the 2-track *single* release-group
(Off the Wall / Get on the Floor) instead of the 10-track album. Because
plan_track_names labels files purely by position, that short tracklist got
mapped onto the full album — track 2 became "Get on the Floor" instead of
"Rock with You", and the real "Off the Wall"/"Get on the Floor" files kept
their streamrip names, yielding duplicate titles.

Extract selection into a pure `_best_release_group()` that breaks title-
similarity ties toward primary-type == "Album". Similarity still dominates;
album preference only settles ties. Live-verified: Off the Wall now resolves
to the 10-track album with track 2 = "Rock With You".

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

worker/lyra_worker/_musicbrainz.py

@@ -9,6 +9,24 @@ def _ratio(a: str, b: str) -> float:
     return SequenceMatcher(None, a.strip().casefold(), b.strip().casefold()).ratio()
 
 
+def _is_album(g: dict) -> bool:
+    return (g.get("primary-type") or "").casefold() == "album"
+
+
+def _best_release_group(album: str, groups: list) -> dict | None:
+    """Choose the best release-group match for `album`. Highest title similarity wins;
+    an Album primary-type breaks ties so a famous album (e.g. Michael Jackson's "Off the
+    Wall") isn't resolved to its same-named single, whose short tracklist would then be
+    mapped positionally onto the full album's files. Returns None below the title threshold.
+    Pure — no network I/O."""
+    if not groups:
+        return None
+    best = max(groups, key=lambda g: (_ratio(album, g.get("title", "")), _is_album(g)))
+    if _ratio(album, best.get("title", "")) < _MIN_TITLE_RATIO:
+        return None
+    return best
+
+
 class MusicBrainzResolver:
     """Real MbResolver using the MusicBrainz webservice via musicbrainzngs.
 
@@ -28,10 +46,8 @@ class MusicBrainzResolver:
 
         res = musicbrainzngs.search_release_groups(query=album, artist=artist, limit=5)
         groups = res.get("release-group-list", [])
-        if not groups:
-            return None
-        rg = max(groups, key=lambda g: _ratio(album, g.get("title", "")))
-        if _ratio(album, rg.get("title", "")) < _MIN_TITLE_RATIO:
+        rg = _best_release_group(album, groups)
+        if rg is None:
             return None
 
         canonical_album = rg.get("title", album)

worker/tests/test_musicbrainz.py

@@ -0,0 +1,36 @@
+from lyra_worker._musicbrainz import _best_release_group
+
+# Offline unit tests for the pure release-group selection logic. The live resolve()
+# 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 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"
+
+
+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"
+
+
+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"
+
+
+def test_below_threshold_returns_none():
+    groups = [_rg("Something Totally Different", "Album")]
+    assert _best_release_group("Off the Wall", groups) is None
+
+
+def test_empty_groups_returns_none():
+    assert _best_release_group("Off the Wall", []) is None