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/unit/test_channel_root.py

@@ -50,7 +50,7 @@ def test_masks_and_minimal_footprint():
 def test_actual_closure_read_conflicts():
     pair, layout, targets = transfer((6, 6, 18, 14))
     validate_channel_transfers(pair, layout, targets)
-    for target in (layout.slot(layout.owner[5, -1]-1, 2),
+    for target in (layout.slot(int(layout.owner[5, -1]) - 1, 2),
                    layout.slot(layout.owner[1, 4], 4),
                    layout.slot(layout.owner[-2, 4], 3)):
         with pytest.raises(ValueError, match="immutable"):