refactor(client): capture send timestamp before socket call

Record the send timestamp immediately before the socket operation to include local enqueue overhead in RTT measurements, providing more conservative latency data for consumers.

crates/irtt-client/src/client.rs

@@ -201,9 +201,8 @@ impl Client {
             payload: vec![],
         };
         let packet = encode_echo_request(&request, self.config.hmac_key.as_deref())?;
-        self.socket.send(&packet)?;
-
         let sent_at = override_ts.unwrap_or_else(ClientTimestamp::now);
+        self.socket.send(&packet)?;
 
         let session = self
             .session