feat(client): implement DSCP negotiation and socket traffic class application

Apply the negotiated DSCP value to the UDP socket after a successful session open and clear it before closing to support network-level quality of service. This change adds platform-specific socket option handling for IPv4 TOS and IPv6 Traffic Class.

crates/irtt-client/src/lib.rs

@@ -8,6 +8,7 @@ mod managed;
 mod probe;
 mod session;
 mod socket;
+mod socket_options;
 mod timing;
 
 pub use client::Client;