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/unit/test_refinement_three_level_multipatch.py

@@ -251,14 +251,14 @@ class ConstructorAndSchedule(unittest.TestCase):
             initial_c=c.current
             def record(name):
                 return lambda source,*a,**kw: events.append((name,source))
-            run._explode.side_effect=record('explode01')
-            run._child.explode.side_effect=record('explode12')
+            run._pairs[0].explode.side_effect=record('explode01')
+            run._pairs[1].explode.side_effect=record('explode12')
             for i,level in enumerate(run._levels):
                 level.kernel.side_effect=record(f'step{i}')
             if bodies:
                 g.boundary.kernel.side_effect=record('bfl2')
-            run._child.coalesce.side_effect=lambda src,dst,*a,**kw: events.append(('restrict21',dst))
-            run._coalesce.side_effect=lambda src,dst,*a,**kw: events.append(('restrict10',dst))
+            run._pairs[1].coalesce.side_effect=lambda src,dst,*a,**kw: events.append(('restrict21',dst))
+            run._pairs[0].coalesce.side_effect=lambda src,dst,*a,**kw: events.append(('restrict10',dst))
             run._status='ready'
             expected=[('explode01',initial_c)]
             for middle in (f.current,f.spare):
@@ -278,13 +278,14 @@ class ConstructorAndSchedule(unittest.TestCase):
             self.assertEqual([l.kernel.call_count for l in run._levels],[1,2,4])
             # A stale-feed implementation must fail the source-identity guard.
             feeds=[]
-            advance=run._advance_child
-            def checked_advance(buffer, **kw):
-                self.assertIs(buffer,(f.current,f.spare)[len(feeds)])
-                feeds.append(buffer)
-                advance(buffer,**kw)
-            with patch.object(run,'_advance_child',side_effect=lambda buffer,**kw:
-                              checked_advance(f.current,**kw)):
+            advance=run._advance
+            def checked_advance(index, buffer, **kw):
+                if index == 2:  # L1 -> L2 feed: must be the L1 hop source
+                    self.assertIs(buffer,(f.current,f.spare)[len(feeds)])
+                    feeds.append(buffer)
+                advance(index,buffer,**kw)
+            with patch.object(run,'_advance',side_effect=lambda index,buffer,**kw:
+                              checked_advance(index,f.current if index == 2 else buffer,**kw)):
                 with self.assertRaises(AssertionError):
                     run.step()
             # A per-region shared-parent advance must fail the 1:2:4 guard.

tests/unit/test_runtime_child_remap.py

@@ -1,4 +1,5 @@
 """CPU mocks execute production remap lifecycle; no GPU numerical claim."""
+from dataclasses import replace
 import unittest
 from unittest.mock import patch
 
@@ -68,8 +69,11 @@ class RuntimeChildRemap(unittest.TestCase):
         run._status = 'ready'
         for bad in ([], [(40,16,48,40)], [(16,16,32,24),(16,27,32,40)]):
             with self.assertRaises(ValueError): run.remap_child(bad)
-        for attribute, value in (('_wall_aware', True), ('child_boundary', object()),
-                                 ('fine_boundary', object()), ('child_patch', None)):
+        hosts, pairs = run._boundary_hosts, run.hierarchy.pairs
+        for attribute, value in (('_wall_aware', True),
+                                 ('_boundary_hosts', [*hosts[:2], object()]),        # child boundary
+                                 ('_boundary_hosts', [hosts[0], object(), hosts[2]]),  # fine boundary
+                                 ('hierarchy', replace(run.hierarchy, pairs=(pairs[0], None)))):
             with patch.object(run, attribute, value):
                 with self.assertRaises(ValueError): run.remap_child(B)
         self.assertEqual(len(allocations), start)

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"):