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_jet_update_failure.py

@@ -102,7 +102,7 @@ def test_simulation_before_initialize_commits_host_only(monkeypatch):
 
 
 def stepper_state(run):
-    boundary = run._device_boundaries[2]
+    boundary = run._boundaries[2]
     return boundary.tables["surface_velocity"].copy(), boundary.jets, run.jets[2]
 
 
@@ -116,8 +116,8 @@ def test_stepper_invalid_input_writes_nothing_and_stays_ready():
             with pytest.raises(error):
                 run.set_jet_amplitude(*args, **kw)
         module.cuda.memcpy_htod.assert_not_called()
-        np.testing.assert_array_equal(run._device_boundaries[2].tables["surface_velocity"], velocity)
-        assert (run._device_boundaries[2].jets, run.jets[2]) == (jets, level_jets)
+        np.testing.assert_array_equal(run._boundaries[2].tables["surface_velocity"], velocity)
+        assert (run._boundaries[2].jets, run.jets[2]) == (jets, level_jets)
         assert run._status == "ready"
         run.set_jet_amplitude(0, .017, level=2)
         assert run.jets[2][0].amplitude == .017
@@ -136,8 +136,8 @@ def test_stepper_device_failure_keeps_host_and_blocks_reuse(where):
             run._stream.synchronize.side_effect = RuntimeError("upload failed")
         with pytest.raises(RuntimeError, match="upload failed"):
             run.set_jet_amplitude(0, .017, level=2)
-        np.testing.assert_array_equal(run._device_boundaries[2].tables["surface_velocity"], velocity)
-        assert (run._device_boundaries[2].jets, run.jets[2]) == (jets, level_jets)
+        np.testing.assert_array_equal(run._boundaries[2].tables["surface_velocity"], velocity)
+        assert (run._boundaries[2].jets, run.jets[2]) == (jets, level_jets)
         assert run._status == "failed"
         module.cuda.memcpy_htod.side_effect = None
         run._stream.synchronize.side_effect = None