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_fine_surface_gpu.py

@@ -140,7 +140,8 @@ def run_patch(speed, *, audit=True):
         build = (patch.object(compiler, "compile_kernel", audit_compiler(compiler.compile_kernel))
                  if audit else nullcontext())
         with build:
-            backend = implementation.FixedPatchStepper(cfg, BOUNDS, context=context, fine_objects=[source])
+            backend = implementation.FixedPatchStepper(cfg, context=context, refinements=[BOUNDS],
+                                             level_objects=[(), (source,)])
         with backend, context.activate():
             c, f = backend._levels
             cl = backend._fine_curved
@@ -325,6 +326,7 @@ def test_cross_interface_donor_rejects_before_cuda():
     context.activate.side_effect = AssertionError("must reject before CUDA activation")
     with patch.object(cuda, "mem_alloc") as allocate:
         with pytest.raises(ValueError, match="donor crosses"):
-            implementation.FixedPatchStepper(cfg, BOUNDS, context=context, fine_objects=[source])
+            implementation.FixedPatchStepper(cfg, context=context, refinements=[BOUNDS],
+                                             level_objects=[(), (source,)])
         context.activate.assert_not_called()
         allocate.assert_not_called()