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.

docs/IRTT_CLIENT_PROTOCOL_SPEC.md

@@ -692,7 +692,8 @@ The following configuration parameters affect protocol behavior:
 
 - **Client send timestamp:** SHOULD be captured as close as possible to the
   send system call. For best RTT accuracy, the timestamp should be taken
-  immediately after the send call returns.
+  immediately before the send call, so that the measured RTT includes any
+  local send/enqueue overhead.
 - **Client receive timestamp:** SHOULD be captured as close as possible to
   the receive system call return, before any packet processing.
 - **Server timestamps:** Determined by the negotiated StampAt parameter: