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_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)