create_order now consults src/lib/crossSell.ts before submitting. When a rule
fires and the user has not answered, it returns NEEDS_CROSS_SELL_ANSWER and
writes nothing; the assistant must ask the question verbatim and retry with
cross_sell_ack. Enforced in the route rather than the prompt so the suggestive
sell cannot be skipped. Skips the prompt when the companion item is already on
the order. Rules are data - add more by appending to CROSS_SELL_RULES.