FixedPatchStepper: exchange metadata on the boundary record; _boundaries name

- _exchange_spacing/_exchange_centers (two more per-level lists, only read
  for levels that have a boundary) become _BlockedBoundary.spacing/centers.
- _level_blocks dropped: remap_finest reads each layout's own .block.
- _device_boundaries -> _boundaries: the record holds host geometry, tables,
  Jets and device handles, so 'device' was misleading (the old host-only
  _boundaries list no longer exists). Tests migrated mechanically.
Remaining per-level lists (_layouts, _level_cfgs, _leaf, _state, _boundaries)
are the immutable construction plan, built before the memory preflight and
kept after close; _levels holds device handles only and is cleared on close.

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

tests/unit/test_runtime_child_remap.py

@@ -70,11 +70,11 @@ class RuntimeChildRemap(unittest.TestCase):
         run._status = 'ready'
         for bad in ([], [(40,16,48,40)], [(16,16,32,24),(16,27,32,55)]):
             with self.assertRaises(ValueError): run.remap_finest(bad)
-        records, pairs = run._device_boundaries, run.hierarchy.pairs
+        records, pairs = run._boundaries, run.hierarchy.pairs
         for attribute, value in (('_wall_aware', True),
                                  # level-2 record whose host geometry differs from objects=()
-                                 ('_device_boundaries', [*records[:2], SimpleNamespace(host=object(), jets=())]),
-                                 ('_device_boundaries', [records[0], object(), records[2]]),  # level-1 body
+                                 ('_boundaries', [*records[:2], SimpleNamespace(host=object(), jets=())]),
+                                 ('_boundaries', [records[0], object(), records[2]]),  # level-1 body
                                  ('hierarchy', replace(run.hierarchy, pairs=(pairs[0], None)))):
             with patch.object(run, attribute, value):
                 with self.assertRaises(ValueError): run.remap_finest(B)