Function run_session_turn 

pub async fn run_session_turn(
    runtime: &mut Runtime,
    messages: &mut Vec<SharedMessage>,
    allowance: &SessionAllowance<'_>,
    role: &TurnBudget,
    totals: &mut SessionTotals,
    session_start: Instant,
    interrupted: &Arc<AtomicBool>,
    on_event: impl FnMut(&StreamEvent, &SessionTotals),
) -> TurnReport

One turn of a watcher session, as synaps agent runs it: the role budget narrowed to what the session has left.