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/client/tests/dscp.rs

@@ -0,0 +1,68 @@
+use super::*;
+use crate::socket_options::socket_traffic_class;
+
+#[test]
+#[cfg(not(any(
+    target_os = "fuchsia",
+    target_os = "redox",
+    target_os = "solaris",
+    target_os = "illumos",
+    target_os = "haiku",
+)))]
+fn normal_open_applies_negotiated_dscp_after_open_and_close_clears_it() {
+    let mut params = default_params();
+    params.dscp = 46;
+    let server = start_fake_server(move |socket, tx| {
+        let (_, peer) = recv_request(&socket, &tx);
+        let reply = open_reply(FLAG_OPEN | FLAG_REPLY, TOKEN, &params, None);
+        socket.send_to(&reply, peer).unwrap();
+        let _ = recv_request(&socket, &tx);
+    });
+    let mut config = default_test_config(server.addr);
+    config.dscp = 46;
+    let mut client = Client::connect(config).unwrap();
+
+    assert_eq!(
+        socket_traffic_class(&client.socket, client.remote).unwrap() & 0xfc,
+        0
+    );
+    assert_open_started(client.open(ClientTimestamp::now()).unwrap());
+    assert_eq!(
+        socket_traffic_class(&client.socket, client.remote).unwrap() & 0xfc,
+        184
+    );
+
+    client.close(ClientTimestamp::now()).unwrap();
+    assert_eq!(
+        socket_traffic_class(&client.socket, client.remote).unwrap(),
+        0
+    );
+    server.join();
+}
+
+#[test]
+#[cfg(not(any(
+    target_os = "fuchsia",
+    target_os = "redox",
+    target_os = "solaris",
+    target_os = "illumos",
+    target_os = "haiku",
+)))]
+fn normal_open_uses_negotiated_dscp_not_requested_dscp() {
+    let mut returned = default_params();
+    returned.dscp = 0;
+    let server = open_success_server(returned);
+
+    let mut config = default_test_config(server.addr);
+    config.dscp = 46;
+    config.negotiation_policy = NegotiationPolicy::Loose;
+    let mut client = Client::connect(config).unwrap();
+
+    let negotiated = assert_open_started(client.open(ClientTimestamp::now()).unwrap());
+    assert_eq!(negotiated.params.dscp, 0);
+    assert_eq!(
+        socket_traffic_class(&client.socket, client.remote).unwrap(),
+        0
+    );
+    server.join();
+}