build(..., origins=None|[(y0, x0), ...]) stores only grid-aligned selected
blocks (unique, strictly row-major, no silent reordering). Cells of absent
blocks own layout.hole (= nblocks*L, one past the last block cell) so an
unguarded index fails rather than wrapping like -1; layout.owned is the
dense mask. Halo pairs never read holes, to_blocked ignores hole values,
to_dense returns NaN there, map_cut_links rejects receivers, solid
neighbors or q<.5 donors in absent blocks, blocked_link_tables stores
owned flags only. origins=None or every origin reproduces the regular
layout exactly. Storage only: no CUDA or stepper changes.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
