test: give the fault-guard fake backend its storage representation

download() now runs _check_storage(), which reads _ddf_shifting and the
per-level configs; the fake backend set neither. Both are set to the
unshifted default, so the download/step failure assertions run again.

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

tests/validation/multigrid/test_refinement_followup.py

@@ -107,6 +107,8 @@ class FaultGuardsCPU(unittest.TestCase):
     def fake_backend(self):
         backend = PeriodicPatchStepper.__new__(PeriodicPatchStepper)
         backend._layouts = [None, None]  # dense storage on both levels
+        backend._ddf_shifting = False  # unshifted on every level, as _check_storage expects
+        backend._level_cfgs = [LBMConfig(nx=16, ny=16, viscosity=.1)] * 2  # ddf_shifting off
         backend.ctx = Mock(activate=nullcontext)
         backend._lock = RLock()
         backend._status = "ready"