feat: MbBrowser seam and FakeMbBrowser test double

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

worker/lyra_worker/adapters/fakes.py

@@ -1,5 +1,6 @@
 from typing import Callable
 
+from lyra_worker.browser import ArtistHit, ReleaseGroupInfo
 from lyra_worker.types import Candidate, DownloadResult, MBTarget, Quality
 
 _QUALITIES = {
@@ -66,3 +67,17 @@ class FailingAdapter(_BaseFake):
         self, candidate: Candidate, dest: str, on_progress: Callable[[float], None]
     ) -> DownloadResult:
         return DownloadResult(ok=False, error="simulated download failure")
+
+
+class FakeMbBrowser:
+    """In-memory MbBrowser for tests. `releases` maps artist mbid -> [ReleaseGroupInfo]."""
+
+    def __init__(self, artists=None, releases=None):
+        self._artists = list(artists or [])
+        self._releases = dict(releases or {})
+
+    def search_artist(self, name: str) -> list[ArtistHit]:
+        return list(self._artists)
+
+    def browse_release_groups(self, artist_mbid: str) -> list[ReleaseGroupInfo]:
+        return list(self._releases.get(artist_mbid, []))