Function rpc_once 

async fn rpc_once(
    holder: &mut WorkerHolder,
    method: &str,
    params: Value,
    deadline: Duration,
) -> RpcOutcome

One RPC with poison-on-drop (§4.4) + rpc_deadline (§4.7). A transport error OR a deadline overrun is a crash (respawn); a worker Failure frame is a domain error (worker alive).