Zero-touch generator: create mode still materializes every variant subset before the cap #2006

Follow-up to #2003 and #2005. #2005 fixed update mode so it stops at the 64th variant without materializing every subset. Its review (comment 6673768309) found that create mode (verify_contract.py:2659-2673) still builds every subset before applying the cap, so it can still grow exponentially in the number of fields. Dispatch mode has no cap but grows linearly.

Done when create mode stops at the cap with the same first variants in the same order (proved by a side-by-side old/new equivalence test like #2005's), plus a bound test that fails on the old code.