Validate physical fine_cfg before surface_link_velocity

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>

tests/validation/multigrid/test_refinement_blocked_fine.py

@@ -74,6 +74,7 @@ def fine_cfg(mode, patch):
     if mode == "slot_reservoir":
         cfg = replace(cfg, y_wall_bc="bounce_back", top_wall_bc="velocity",
                       wall_slot_width=0, wall_normal_velocity=0.0, velocity=0.0)
+    cfg.validate()  # as the stepper does before surface_link_velocity
     return cfg