Seed-based storage: keep the selected variant in full, store only seeds for unselected stages
Change replay JSON storage to 'store the recipe, not the cake'. Per generation, save the SELECTED variant's full design parameters, plus only the SEED (variant_seed) for each UNSELECTED stage. Any sibling can then be regenerated identically on demand from parent + seed, so full backtracking into non-selected stages is preserved at near-zero storage cost - scaling cleanly from 3x3 to nxn (e.g. 12x12). REQUIRES generation to be bit-for-bit deterministic from seed+parent, so a 'genome_version' is stamped into the JSON and reconstruction always uses the matching algorithm version.