Preview, then launch or pause the measured Buyer Agent test

Two steps. First call (no confirmation) returns 409 confirmation_required with the complete behavior, its preview hash and the required fields. Pausing (state paused) is then confirmed with literal user_confirmed true and confirmed_preview_hash. Starting a test, or changing who is in a running one (state live_test), changes what visitors see, so the first answer also carries visitor_test: every line of text a visitor can be shown, the pages, the button text and the share of visitors, with its own hash. Show the owner or admin that exact text first. The second call must then send literal user_confirmed true, confirmed_preview_hash and confirmed_proposal_hash (the visitor_test hash). A missing or changed hash answers 409 confirmation_stale with the current text and starts nothing. Starting requires an owner or admin; the confirmed rows are recorded as approved by that person in the same step. Readiness, billing, role and caps are rechecked on every call.