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-stats/src/normalization.rs

@@ -203,7 +203,7 @@ fn duration_ns_i128(duration: Duration) -> i128 {
 }
 
 fn signed_duration_ns(duration: SignedDuration) -> i128 {
-    duration.ns
+    duration.as_nanos()
 }
 
 #[cfg(test)]