Migrate current callers to level-indexed refinement API

tests/validation/multigrid/test_refinement_gpu.py

@@ -91,8 +91,8 @@ class CoupledGPU(unittest.TestCase):
                                     ddf_shifting=shifted)
                     with FixedPatchStepper(cfg, context=context, refinements=[(4, 4, 12, 12)],
                                            **storage) as backend:
-                        self.assertAlmostEqual(backend.patch.tau_coarse, spec.tau_coarse)
-                        self.assertAlmostEqual(backend.patch.tau_fine, spec.tau_fine)
+                        self.assertAlmostEqual(backend.hierarchy.pairs[0].tau_coarse, spec.tau_coarse)
+                        self.assertAlmostEqual(backend.hierarchy.pairs[0].tau_fine, spec.tau_fine)
                         self.assertEqual(tuple(level.shape for level in backend._levels),
                                          ((9, 16, 16), (9, 24, 24)))
                         # Physical FP64 g is encoded by initialize before FP32 rounding.
@@ -101,7 +101,7 @@ class CoupledGPU(unittest.TestCase):
                         backend.step(1)
                         actual = backend.download()
                         self.assertEqual(backend.step_count, 1)
-                        backend.patch.check_synchronization(
+                        backend.hierarchy.pairs[0].check_synchronization(
                             1, next_state.tick, phase="postcollision_g")
                         for got, want, arrival in zip(actual, expected, arrivals):
                             np.testing.assert_array_equal(np.isnan(got), np.isnan(want))
@@ -165,13 +165,13 @@ class CoupledGPU(unittest.TestCase):
                     with self.assertRaisesRegex(RuntimeError, "failed"):
                         operation()
                 raw = level_storage(fine)
-                self.assertTrue(np.isfinite(raw.ravel()[backend.patch.coalesce_fine]).all())
+                self.assertTrue(np.isfinite(raw.ravel()[backend.hierarchy.pairs[0].coalesce_fine]).all())
             self.assertFalse(context.is_closed)
         print("partial failure: two fine hops completed, no coarse step published; cleanup PASS", flush=True)
 
     def eight_steps(self, backend, spec, maps):
         """Rest and seeded nonequilibrium, 8 steps against the scalar reference."""
-        p = backend.patch
+        p = backend.hierarchy.pairs[0]
         for case in ("rest", "nonequilibrium_seed1"):
             with self.subTest(case=case):
                 state = ref.make_state(spec, velocity=(0, 0) if case == "rest" else (.03, -.02))