feat: link only source paths a tree confirms

The deploy has no tree cache, so a citation it could not check was
linked into the declared repository: 30 paths answered 404. The paths
held at each pin are now recorded in website/src/data/source-paths.json
by python scripts/sitesources.py, and a path absent from it renders as
code. An address inside a shipped binary reads as one.

tests/test_source_ref_targets.py

@@ -41,7 +41,7 @@ class BinaryContinuationsAreNotLinked(unittest.TestCase):
 
     def _render(self, value: str) -> str:
         original = sitesources._tree_paths
-        sitesources._tree_paths = lambda repo, pin: None  # type: ignore[assignment]
+        sitesources._tree_paths = lambda repo, pin: frozenset({"FPLoader.cpp"})  # type: ignore[assignment]
         try:
             return sitesources._source_ref_markdown(self.PROFILE, value)
         finally: