refactor(client): use SignedDuration for one-way delay to support negative values

Allow one-way delay samples to represent negative values caused by wall-clock skew between client and server. Update CLI output and statistics collection to correctly handle and report these signed durations.

crates/irtt-client/src/client.rs

@@ -789,11 +789,11 @@ fn compute_one_way(
     let c2s = server_recv_wall
         .zip(client_send_ns)
         .and_then(|(srv, cli)| srv.checked_sub(cli))
-        .and_then(|d| u64::try_from(d).ok().map(Duration::from_nanos));
+        .map(|d| SignedDuration::from_nanos(i128::from(d)));
     let s2c = client_recv_ns
         .zip(server_send_wall)
         .and_then(|(cli, srv)| cli.checked_sub(srv))
-        .and_then(|d| u64::try_from(d).ok().map(Duration::from_nanos));
+        .map(|d| SignedDuration::from_nanos(i128::from(d)));
 
     if c2s.is_none() && s2c.is_none() {
         return None;