Starts a Motion session WITHOUT the user having a tab open. Use this when they have no pairing code — you can create and edit compositions immediately, and give them the returned link when they want to watch. Anything needing to SEE the result (inspect_composition) or touching their media still requires them to open it: send them the link, then call wait_for_tab rather than asking them to report back. The scene is kept for 24 hours after the last change.