Migrate stepper tests to the level-list representation

Schedule test drives _pairs[i].explode/coalesce and patches _advance(index,
buffer) for the stale L1->L2 feed mutant; remap rejection test patches
_boundary_hosts entries and hierarchy.pairs instead of the read-only views;
the GPU launch-failure test patches _pairs[0].explode. Assertions unchanged.

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

tests/validation/multigrid/test_refinement_gpu.py

@@ -194,7 +194,7 @@ class CoupledGPU(unittest.TestCase):
                               f"max_corner_packet={max_corner:.9g}; ownership/counts PASS", flush=True)
 
                 # Fail before any additional time step; no stale synchronized export.
-                with patch.object(backend, "_explode", side_effect=RuntimeError("injected launch failure")):
+                with patch.object(backend._pairs[0], "explode", side_effect=RuntimeError("injected launch failure")):
                     with self.assertRaisesRegex(RuntimeError, "injected"):
                         backend.step()
                 with self.assertRaisesRegex(RuntimeError, "failed"):