refactor(client): use SignedDuration for effective RTT to support negative values

Simplify RttSample by merging signed and unsigned fields into a single SignedDuration for adjusted and effective RTT. This ensures accurate reporting and statistics when server processing time exceeds raw RTT.

crates/irtt-cli/src/summary.rs

@@ -206,10 +206,8 @@ mod tests {
             received_at,
             rtt: RttSample {
                 raw: Duration::from_micros(1500),
-                adjusted: Some(Duration::from_micros(1200)),
-                effective: Duration::from_micros(1200),
-                adjusted_signed: Some(SignedDuration { ns: 1_200_000 }),
-                effective_signed: SignedDuration { ns: 1_200_000 },
+                adjusted: Some(SignedDuration { ns: 1_200_000 }),
+                effective: SignedDuration { ns: 1_200_000 },
             },
             server_timing: Some(ServerTiming {
                 receive_wall_ns: None,
@@ -254,10 +252,8 @@ mod tests {
             received_at,
             rtt: RttSample {
                 raw: Duration::from_micros(1500),
-                adjusted: Some(Duration::from_micros(1200)),
-                effective: Duration::from_micros(1200),
-                adjusted_signed: Some(SignedDuration { ns: 1_200_000 }),
-                effective_signed: SignedDuration { ns: 1_200_000 },
+                adjusted: Some(SignedDuration { ns: 1_200_000 }),
+                effective: SignedDuration { ns: 1_200_000 },
             },
             server_timing: None,
             one_way: None,