fix(client): avoid lossy timing casts

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

@@ -1479,6 +1479,38 @@ fn compute_one_way_returns_none_when_both_directions_fail() {
     assert!(result.is_none());
 }
 
+#[test]
+fn unix_epoch_ns_i64_rejects_out_of_range_wall_time() {
+    let i64_max_ns = u64::try_from(i64::MAX).unwrap();
+    let max = SystemTime::UNIX_EPOCH + Duration::from_nanos(i64_max_ns);
+    let overflow = max + Duration::from_nanos(1);
+
+    assert_eq!(unix_epoch_ns_i64(max), Some(i64::MAX));
+    assert_eq!(unix_epoch_ns_i64(overflow), None);
+}
+
+#[test]
+fn compute_one_way_omits_samples_when_client_wall_time_overflows_i64() {
+    let i64_max_ns = u64::try_from(i64::MAX).unwrap();
+    let overflow_wall =
+        SystemTime::UNIX_EPOCH + Duration::from_nanos(i64_max_ns) + Duration::from_nanos(1);
+    let sent_at = ClientTimestamp {
+        mono: Instant::now(),
+        wall: overflow_wall,
+    };
+    let received_at = ClientTimestamp {
+        mono: sent_at.mono + Duration::from_millis(40),
+        wall: overflow_wall + Duration::from_millis(40),
+    };
+    let ts = TimestampFields {
+        recv_wall: Some(i64::MAX),
+        send_wall: Some(i64::MAX),
+        ..Default::default()
+    };
+
+    assert_eq!(compute_one_way(&sent_at, &received_at, &ts), None);
+}
+
 #[test]
 fn matched_reply_with_reversed_monotonic_time_still_emits_event() {
     let params = default_params();