The blocked actuated path called the helper with an unvalidated fine config (nq=0), so receiver*nq+dir collapsed distinct writers and the unique-writer guard fired. Validate after wall-aware sanitization, as the dense path does before assign_surface; the test's fine_cfg helper too. Unique-writer guard unchanged. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>