Sample crystal structures with CrystaLLM for GenBench

GenBench's sample contract with CrystaLLM (small checkpoint): formula prompts the cell composition with Z of 1, 2, 3, 4, 6 or 8 formula units (up to 40 atoms), chemical_system draws one formula per sample from the system's stoichiometries, and unconditional starts from data_ alone. All samples generate in one batch. Returns post-processed CIFs inline as JSON, dropping any that do not parse; creates no Ouro assets.