test: update stale fixtures for current sources (no src change)

- test_channel_root: owner is uint64; int() before -1 so the probe slot
  stays an integer under NumPy 1 value-based promotion.
- test_shifted_storage: build FP32 fixtures with FP32 NaN; np.where with a
  Python NaN promoted to FP64 under NumPy 1, so the shifted FP32 contract
  rejected the fixture before the intended checks.
- test_block_stepper: fake backend sets _ddf_shifting (unshifted).
- test_fine_surface_gpu: set _boundary_hosts/_level_jets; fine_boundary
  is now a read-only view over the level lists.
- test_refinement_followup: set _layouts (fine_layout is a read-only
  view); a pair tau that disagrees with its level config is now rejected
  by hierarchy._check_chain before the per-level omega/tau check.

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

tests/validation/multigrid/test_block_stepper.py

@@ -192,6 +192,7 @@ class CpuContracts(unittest.TestCase):
         backend._lock = RLock()
         backend._status = "ready"
         backend._step_count = 0
+        backend._ddf_shifting = False  # unshifted storage, as for the default config
         backend._stream = Mock()
         backend._module = Mock()
         backend._copy = backend._step = Mock()