feat: add SourceAdapter contract and fake adapters

worker/lyra_worker/adapters/base.py

@@ -0,0 +1,23 @@
+from typing import Callable, Protocol, runtime_checkable
+
+from lyra_worker.types import Candidate, DownloadResult, MBTarget
+
+
+@runtime_checkable
+class SourceAdapter(Protocol):
+    name: str
+    tier: int
+
+    def health(self) -> bool:
+        """Is this source configured and reachable?"""
+        ...
+
+    def search(self, target: MBTarget) -> list[Candidate]:
+        """Return zero or more candidates matching target. No downloading."""
+        ...
+
+    def download(
+        self, candidate: Candidate, dest: str, on_progress: Callable[[float], None]
+    ) -> DownloadResult:
+        """Fetch candidate into dest, reporting progress 0.0..1.0."""
+        ...