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>
