Asserts 21 aperture links, all with nonzero velocity, and 3 halo-writer plus 2 halo-donor aperture rows in each decomposed layout (actuation coverage across seams, not only geometric cut-link coverage). Velocity is given in coarse units; prepare_fine_boundary scales it once. The GPU gate remains storage equivalence; no zero-mass-change claim. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>