Check what's still missing before submitting

Evaluates the draft submission and everything uploaded so far against the requirement set, and tells you what's still outstanding while it can all still be edited. This is the difference between one clean submission and a rejection round trip.

It changes nothing, and it's the one documented exception to the idempotency rule. It takes no Idempotency-Key, is never replayed, and every call returns a fresh evaluation of the current draft. Sending a key is ignored rather than rejected.