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The fourth section is one 8-byte (count, offset) record per grid cell, followed by the 32-byte records it points at. The check: the lowest offset any cell refers to equals align16(offsets[3] + cells*8) on 11 of 11 objects - and the alignment term is visible rather than assumed because the three 5x5x5 maps have 125*8 = 1000 bytes of index, which is not 16-aligned, so their payload starts 8 bytes later than the six 10x10x10 maps'. Two further invariants from the same sweep: every occupied cell has count exactly 1 (total items == occupied cells on all 11, so it is one record per cell rather than a bucket list), and counts[4] equals occupied cells + 2 exactly on all 11 - the +2 unexplained and recorded as such. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01PMRJjbxLqZtsb5Vb7KunPE