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/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