feat: MbBrowser seam and FakeMbBrowser test double

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

worker/lyra_worker/browser.py

@@ -0,0 +1,28 @@
+from dataclasses import dataclass, field
+from typing import Protocol
+
+
+@dataclass(frozen=True)
+class ArtistHit:
+    mbid: str
+    name: str
+    disambiguation: str = ""
+
+
+@dataclass(frozen=True)
+class ReleaseGroupInfo:
+    rg_mbid: str
+    title: str
+    primary_type: str = ""
+    secondary_types: tuple[str, ...] = ()
+    first_release_date: str = ""
+
+
+class MbBrowser(Protocol):
+    def search_artist(self, name: str) -> list[ArtistHit]:
+        """Artist name search → ranked hits (best first)."""
+        ...
+
+    def browse_release_groups(self, artist_mbid: str) -> list[ReleaseGroupInfo]:
+        """All official release-groups for an artist MBID."""
+        ...