FixedPatchStepper: one boundary/Jet record per level, host preparation helpers

- Jets and host boundary geometry are views of the per-level _BlockedBoundary
  record (drop duplicate _level_jets/_boundaries registration).
- Host preparation split into _level_layouts, _host_boundaries, _fluid_leaves
  (numpy only, before any device work); constructor order unchanged.
- set_jet_amplitude: explicit prepare -> device write -> commit; behaviour
  unchanged (invalid input raises before any change, uncertain write fails closed).
- Pair accessor: FixedPatch2D.rectangles, FixedPatchSet2D.rectangles and
  fine_level_offset=(0, 0); removes two incidental isinstance branches.
  Geometry/representation unchanged; scalar/set types stay distinct.
- @property read-only views; blocks.py responsibility docstring corrected.
- Migrate test_runtime_child_remap to the public boundaries view.

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

tests/unit/test_runtime_child_remap.py

@@ -69,7 +69,7 @@ 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)
-        hosts, pairs = run._boundaries, run.hierarchy.pairs
+        hosts, pairs = run.boundaries, run.hierarchy.pairs
         for attribute, value in (('_wall_aware', True),
                                  ('_boundaries', [*hosts[:2], object()]),        # child boundary
                                  ('_boundaries', [hosts[0], object(), hosts[2]]),  # fine boundary