Function serve_transcript_socket 

pub async fn serve_transcript_socket(
    socket: WebSocket,
    state: &ServerState,
    caller: &CallerIdentity,
    subscription: &TranscriptSubscription,
) -> Result<(), ServerError>

Serve a transcript subscription on an already-upgraded socket.

Flow: decode + namespace-gate FIRST (anti-leak: a denied caller receives one terminal wire-error frame + close, byte-identical to the workflow path); then attach the live broadcast BEFORE reading the durable O tail (gap-free splice); replay the durable tail; forward live events until the client closes or the subscriber lags (one typed transcript_lagged frame then close).

Errors

Returns ServerError when decode/authorization fails (after the terminal frame is sent), the durable replay read fails, or the stream ends with a lag terminal frame.