P1 (5/n): six GPU validation tests use the level-list constructor

test_refinement_gpu (incl. PeriodicPatchStepper alias calls),
test_channel_root_gpu, test_fine_surface_gpu, test_shifted_remap_gpu,
test_runtime_child_remap_gpu and test_refinement_three_level_multipatch_gpu
pass refinements/level_* lists; scalar and set children map to the second
refinement entry exactly as _legacy_levels did. Assertions unchanged.

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

tests/validation/multigrid/test_channel_root_gpu.py

@@ -106,9 +106,10 @@ class ChannelRootGPU(unittest.TestCase):
                     cfg = LBMConfig(nx=48, ny=32, viscosity=.1, velocity=.015, ddf_shifting=shifted)
                     reference = None
                     for block in ((32,48), (8,8), (4,1)):
-                        with FixedPatchStepper(cfg, (8,6,40,26), context=context,
-                                coarse_boundary="channel", base_block=block, fine_block=(8,8),
-                                fine_objects=(body,) if bodies else (), jets=(jet,) if bodies else ()) as backend:
+                        with FixedPatchStepper(cfg, context=context, coarse_boundary="channel",
+                                refinements=[(8,6,40,26)], level_blocks=[block, (8,8)],
+                                level_objects=[(), (body,) if bodies else ()],
+                                level_jets=[(), (jet,) if bodies else ()]) as backend:
                             arrays = [state(*level.shape[1:]) for level in backend._levels]
                             backend.initialize(*arrays)
                             backend.step(4)