Several separated (>= 4 coarse cells) body-free 2:1 patches share one
level-wide fine grid stored by a sparse BlockLayout2D over the patch
supports. FixedPatchSet2D re-addresses each FixedPatch2D's tables to level
coordinates (same rows/child order, canonical patch order). The stepper
accepts a sequence of rectangles as bounds (requires fine_block and
base_block; rejects fine_objects/jets), reusing the existing blocked step
sequence and kernels. Helpers use a covered mask instead of one rectangle
and skip unstored fine holes. No kernel changes.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
