async fn ack_envelope(
write: &mut SplitSink<WebSocketStream<MaybeTlsStream<TcpStream>>, Message>,
envelope_id: &str,
) -> Result<(), String>Expand description
Sends {"envelope_id": "..."} back on the socket the envelope arrived
on. Kept allocation-light and infallible-as-possible per spec §2.2’s
note that the ACK path should stay cheap and unconditional.