feat(cli): add support for configuring outgoing TTL and hop limit

Introduce a `--ttl` flag to allow users to set the IPv4 TTL or IPv6 hop limit, providing control over packet lifespan and helping diagnose network topology issues.

crates/irtt-client/src/client/tests/mod.rs

@@ -17,3 +17,4 @@ mod negotiation;
 mod no_test;
 mod open;
 mod probes_replies;
+mod ttl;

crates/irtt-client/src/client/tests/ttl.rs

@@ -0,0 +1,49 @@
+use crate::socket_options::socket_ttl;
+
+use super::*;
+
+#[test]
+fn connect_accepts_valid_ttl_config_values() {
+    for ttl in [None, Some(1), Some(255)] {
+        let mut config = default_test_config(SocketAddr::from(([127, 0, 0, 1], 9)));
+        config.socket_config.ttl = ttl;
+        Client::connect(config).unwrap();
+    }
+}
+
+#[test]
+fn connect_applies_configured_ipv4_ttl_before_open_and_close_preserves_it() {
+    let params = Params {
+        protocol_version: PROTOCOL_VERSION,
+        duration_ns: 3_000_000_000,
+        interval_ns: 1_000_000_000,
+        received_stats: ReceivedStats::Both,
+        stamp_at: StampAt::Both,
+        clock: Clock::Both,
+        ..Params::default()
+    };
+    let server = open_success_server(params.clone());
+    let mut config = default_test_config(server.addr);
+    config.socket_config.ttl = Some(64);
+    let mut client = Client::connect(config).unwrap();
+
+    assert_eq!(socket_ttl(&client.socket, client.remote).unwrap(), 64);
+
+    client.open(ClientTimestamp::now()).unwrap();
+    server.join();
+
+    client.close(ClientTimestamp::now()).unwrap();
+    assert_eq!(socket_ttl(&client.socket, client.remote).unwrap(), 64);
+}
+
+#[test]
+fn connect_rejects_invalid_ttl_values() {
+    for ttl in [0, 256] {
+        let mut config = default_test_config(SocketAddr::from(([127, 0, 0, 1], 9)));
+        config.socket_config.ttl = Some(ttl);
+        assert!(matches!(
+            Client::connect(config),
+            Err(ClientError::InvalidConfig { .. })
+        ));
+    }
+}