refactor(client): encapsulate SignedDuration and enhance its API

Hide internal fields to improve type safety and provide helper methods for unit conversions and negative value checks. Fix several feature-gated test compilation issues.

crates/irtt-client/src/client/tests/probes_replies/timing_math.rs

@@ -18,8 +18,8 @@ fn server_processing_greater_than_raw_does_not_underflow() {
             ..Default::default()
         },
     );
-    assert_eq!(rtt.adjusted, Some(SignedDuration { ns: -999_999_999 }));
-    assert_eq!(rtt.effective, SignedDuration { ns: -999_999_999 });
+    assert_eq!(rtt.adjusted, Some(SignedDuration::from_nanos(-999_999_999)));
+    assert_eq!(rtt.effective, SignedDuration::from_nanos(-999_999_999));
 }
 
 #[test]