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_multiple_pinned_remap.py

@@ -96,11 +96,11 @@ class MultiplePinnedRemap(unittest.TestCase):
         before = run.download()
         clone = run.remap_finest((PINS+MOVED+ADDED)[::-1], objects=bodies)
         try:
-            self.assertEqual(tuple(j.amplitude for j in clone._device_boundaries[2].jets), (.017, -.011))
-            np.testing.assert_array_equal(clone._device_boundaries[2].tables['surface_velocity'],
-                                          run._device_boundaries[2].tables['surface_velocity'])
+            self.assertEqual(tuple(j.amplitude for j in clone._boundaries[2].jets), (.017, -.011))
+            np.testing.assert_array_equal(clone._boundaries[2].tables['surface_velocity'],
+                                          run._boundaries[2].tables['surface_velocity'])
             clone.set_jet_amplitude(1, .005, level=2)
-            self.assertEqual(run._device_boundaries[2].jets[1].amplitude, -.011)
+            self.assertEqual(run._boundaries[2].jets[1].amplitude, -.011)
             clone.step()
             self.assertEqual((run.step_count, clone.step_count), (7, 8))
             for got, want in zip(run.download(), before):