Save registration details, start checkout or check reserved submission availability (billing:write, business owner only).

action 'save' mirrors save_text_registration_details: pass only the fields the owner gave; the rest is filled from what is on file, and while questions remain the response lists saved_steps, missing and next_question (code needs_answers). action 'start_checkout' mirrors start_text_registration_checkout: it returns checkout_url for the owner to pay (nothing is charged here), or included_in_plan and filed when the plan includes registration. The EIN is never accepted over the API (400 ein_not_accepted); the owner types it on setup_url. Writes need the key user to be the business owner (403 owner_required). start_checkout requires confirm_submission true; included plans file immediately. submit is disabled with 503 submission_unavailable pending shared durable reservation and reconciliation; it performs no filing. Correcting saved details does not resubmit rejected provider objects, which need operator reconciliation. Poll GET for state: submission is not approval. No document contents, credentials or legal attestations are accepted.