Source (write) and reverse-donor addresses are now the receiver's local
core address +/- (ey*local_nx + ex), i.e. the window image the next local
pull reads. block_cell preferred the in-window core image on a one-block
periodic axis, so BFL wrote a copy the pull never reads. Physical
neighbors are still formed only to reject non-periodic exteriors.
block_cell semantics unchanged; docstring notes the image ambiguity.
Adds an end-to-end synthetic seam test (x=0/x=nx-1/corner wraps) on
one-block-x, one-block-y and 1x1 periodic layouts.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Each physical link belongs once to the block owning its fluid receiver
core; the reflected source population is written into that block's own
core/halo copy (no halo copy between BFL and pull). Rejects repeated
links and repeated local writers; one row per link so force counts once.
Documents stale remote solid-core slots vs unique core ownership, and
that the 4-cell refinement halo is separate. Static geometry only.
Tests: address equivalence incl. cross-block q<.5 donors, and static
BFL+pull bit-equal to the dense operator on fluid cells.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
