Function handle_sub

async fn handle_sub(
    session: &Session,
    subs: &mut HashMap<String, LiveSub>,
    out_tx: &Sender<ServerOp>,
    verbose: bool,
    sid: String,
    message_name: String,
    topic: String,
) -> Result<(), Fatal>

Subscribes the engine and spawns a forwarder that writes MSG frames.

Duplicate sid or the per-connection cap is -ERR without closing. The protocol has no resource-limit code; the cap uses Illegal-State.