feat(client): add timestamp to ClientEvent::Warning

Include a timestamp when reporting warnings to allow consumers like the machine-readable formatter to provide temporal context for troubleshooting.

crates/irtt-client/src/event.rs

@@ -96,6 +96,7 @@ pub enum ClientEvent {
     Warning {
         kind: WarningKind,
         message: String,
+        at: ClientTimestamp,
     },
 }