Address PR #345 CI review P3s: refresh stale comment + add removal regression guard

- tests/test_rust_backend.py: updated the stale "tracked as a low-priority
  Rust cleanup TODO" comment to reflect that the Rust-side removal is now
  done alongside the Python-side removal.
- tests/test_rust_backend.py: new test_compute_synthetic_weights_is_removed
  as a regression guard against accidental re-export of the deleted PyO3
  binding. Asserts both `from diff_diff._rust_backend import
  compute_synthetic_weights` raises ImportError and the attribute is
  absent on the module object.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

tests/test_rust_backend.py

@@ -106,8 +106,26 @@ def test_bootstrap_different_seeds(self):
 
     # Tests for `compute_synthetic_weights` direct Rust binding removed in
     # the silent-failures audit post-cleanup (finding #22). The helper was
-    # deleted from the Python layer; remaining Rust symbol is dead code
-    # (tracked as a low-priority Rust cleanup TODO).
+    # deleted from the Python layer and the Rust symbol was subsequently
+    # removed from `rust/src/weights.rs` + unregistered in `rust/src/lib.rs`.
+
+    def test_compute_synthetic_weights_is_removed(self):
+        """Regression guard against accidental re-export of the deleted
+        `compute_synthetic_weights` PyO3 binding (silent-failures finding
+        #22). If this test fails, someone reintroduced the binding — audit
+        the reason before adding it back."""
+        import diff_diff._rust_backend as rb
+
+        with pytest.raises(ImportError):
+            from diff_diff._rust_backend import (  # noqa: F401
+                compute_synthetic_weights,
+            )
+
+        assert not hasattr(rb, "compute_synthetic_weights"), (
+            "compute_synthetic_weights was removed from the Rust backend "
+            "in the post-audit cleanup for finding #22; its presence here "
+            "indicates accidental re-export."
+        )
 
     # =========================================================================
     # Simplex Projection Tests