Keep the new stub's docstring to one line

The reviewer's round 2 verified all nine round-1 corrections by re-measuring
them, then failed the commit on the one thing it added that the repo's own bar
forbids: a two-line docstring on _TinyDense. p1-6.4 still lists "comments to one
line" as open work, so a new commit should not widen it. The 470MB rationale
already lives in CHECKLIST.md.

Also cites the CI run for the commit under review (37744547992 @ 20c0416, also
success) instead of stopping at the previous one.

bench/tests/test_joint.py

@@ -37,8 +37,7 @@ def _queries() -> list[str]:
 
 
 class _TinyDense:
-    """Stand-in embedding model: these tests ask whether two harness paths agree, not
-    whether embeddings are good, so they must run without a 470MB download."""
+    """Stand-in embedding model: these tests compare two harness paths, not embedding quality."""
 
     def encode(self, texts: Sequence[str], **_: Any) -> np.ndarray:
         return np.stack([_unit_vector(str(text)) for text in texts]).astype(np.float32)