feat(discovery): SimilaritySource protocol + FakeSimilaritySource

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

worker/lyra_worker/adapters/fakes.py

@@ -2,6 +2,7 @@ from typing import Callable
 
 from lyra_worker.browser import ArtistHit, ReleaseGroupInfo
 from lyra_worker.probe import ProbeResult
+from lyra_worker.similarity.base import SimilarArtist
 from lyra_worker.types import Candidate, DownloadResult, MBTarget, Quality
 
 _QUALITIES = {
@@ -90,3 +91,19 @@ class FakeAudioProbe:
         self._r = ProbeResult(artist=artist, album=album, fmt=fmt, quality_class=quality_class)
     def probe(self, path: str) -> ProbeResult:
         return self._r
+
+
+class FakeSimilaritySource:
+    """In-memory SimilaritySource for tests. `similar` maps seed mbid -> [SimilarArtist]."""
+
+    name = "fake"
+
+    def __init__(self, similar=None, healthy=True):
+        self._similar = dict(similar or {})
+        self._healthy = healthy
+
+    def health(self) -> bool:
+        return self._healthy
+
+    def similar_artists(self, mbid: str) -> list[SimilarArtist]:
+        return list(self._similar.get(mbid, []))

worker/lyra_worker/similarity/base.py

@@ -0,0 +1,22 @@
+from dataclasses import dataclass
+from typing import Protocol, runtime_checkable
+
+
+@dataclass(frozen=True)
+class SimilarArtist:
+    mbid: str
+    name: str
+    score: float
+
+
+@runtime_checkable
+class SimilaritySource(Protocol):
+    name: str
+
+    def health(self) -> bool:
+        """Is this source reachable right now?"""
+        ...
+
+    def similar_artists(self, mbid: str) -> list[SimilarArtist]:
+        """Artists similar to the given artist MBID (MBID-native)."""
+        ...