Mark wall rows OUTER in a zero-refinement hierarchy

build_hierarchy(cfg, [], wall_aware=True) left both base wall rows ACTIVE;
the refined path already marked them OUTER. Add the regression test.
Found by native review of eeb90ac.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/CelerisLab/lbm/hierarchy.py

@@ -62,7 +62,10 @@ def build_hierarchy(cfg, refinements, *, wall_aware=False):
     if pairs:
         kinds.append(pairs[-1].fine_kind)
     else:
-        kinds.append(np.full((cfg.ny, cfg.nx), ACTIVE, np.uint8))
+        kind = np.full((cfg.ny, cfg.nx), ACTIVE, np.uint8)
+        if wall_aware:
+            kind[[0, -1]] = OUTER_GHOST
+        kinds.append(kind)
     return Hierarchy(tuple(configs), tuple(pairs), tuple(kinds))