Function run_turn_child 

pub fn run_turn_child(
    payload: &SpawnPayload,
    intel: &IntelClient,
    servers: &[McpClient],
    up: &Arc<Mutex<Stdout>>,
    cancel: &Arc<AtomicBool>,
    replies: &Arc<Replies>,
    log: &Logger,
) -> i32

The process entry for a Role::Turn child: run the turn, report Usage + TurnDone, return the exit code.