Fixed-size 2D blocks, block-contiguous Q-major storage, explicit per-axis periodicity; non-periodic exterior halo cells stay unpaired (NaN). Provides owner/halo tables for a later periodic GPU consumer. Cut-link mapping deliberately omitted until the BFL write policy is settled. Tests: ownership, pair invariants, pack round trip, tagged pull vs dense. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>