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>
