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/validation/multigrid/test_shifted_jets_gpu.py

@@ -102,7 +102,7 @@ class Ledger:
 
     def step(self):
         run=self.run;self.samples.clear();run.step()
-        b=run._device_boundaries[self.index];h=run._exchange_spacing[self.index]
+        b=run._boundaries[self.index];h=b.spacing
         self.test.assertEqual(len(self.samples),2**self.index)
         expected=np.zeros_like(self.samples[0],dtype=np.float64)
         weights=np.array([h*h,h*h,h*h*h,h*h])
@@ -140,7 +140,7 @@ class ShiftedJetsGPU(unittest.TestCase):
                         level_objects=[(),(obj,)],level_jets=[(),(whole,)],observe_raw_exchange=True) as run:
                     run._initialize_raw(initial(run,rho,u=.013,v=0.))
                     expected=run._download_raw();ledger=Ledger(self,run,1)
-                    self.assertTrue(run._device_boundaries[1].tables['aperture'].all())
+                    self.assertTrue(run._boundaries[1].tables['aperture'].all())
                     for _ in range(4):ledger.step()
                     error=max(float(abs(a[:,leaf]-b[:,leaf]).max()) for a,b,leaf in
                               zip(run._download_raw().values,expected.values,run.leaf))
@@ -201,7 +201,7 @@ class ShiftedJetsGPU(unittest.TestCase):
             self.exact_raw(before,old._download_raw())
             self.exact_raw(_StoredPopulations(expected.g,True),new._download_raw())
             self.assertEqual(new.jets[2][0].amplitude,-.001)
-            np.testing.assert_array_equal(new._device_boundaries[2].tables['surface_velocity'],old._device_boundaries[2].tables['surface_velocity'])
+            np.testing.assert_array_equal(new._boundaries[2].tables['surface_velocity'],old._boundaries[2].tables['surface_velocity'])
             newledger=Ledger(self,new,2);ledger.step();newledger.step()
             self.assertEqual((old.step_count,new.step_count),(count+1,count+1))
             ledger.report('L2 source');newledger.report('L2 remapped')