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_fine_surface_gpu.py

@@ -281,7 +281,8 @@ def test_fine_surface_four_hops(speed):
 @pytest.mark.parametrize("missing", ["surface_velocity_gpu", "link_exchange_gpu"])
 def test_missing_surface_device_buffer_rejects_before_launch(missing):
     backend = implementation.FixedPatchStepper.__new__(implementation.FixedPatchStepper)
-    backend.fine_boundary = SimpleNamespace(objects=[object_spec(.002)])
+    backend._boundary_hosts = [None, SimpleNamespace(objects=[object_spec(.002)])]
+    backend._level_jets = [(), ()]
     backend._fine_curved = SimpleNamespace(fluid_idx_gpu=object(),
                                           surface_velocity_gpu=object(), link_exchange_gpu=object())
     setattr(backend._fine_curved, missing, None)