Target steering or an interrupt to this Run [write]

For a Run of a catalog handle, one user.message starts the next turn of the Run's session as a new Run and is answered with { run_id, status } naming that new Run (also in Location); it follows the idempotency rules of starting a Run, and is refused with 409 session_busy while a turn of the session is still going and 409 session_archived once the session has ended. One user.interrupt stops this Run's turn and is answered with { run_id, status } for this Run; a Run that has already ended is answered with the status it ended in. A system message is not accepted.