fix(client): use kernel transmit time for upstream OWD

Upstream one-way delay now prefers the correlated Linux TX_SOFTWARE wall
timestamp retained by PendingProbe when it is locally plausible (sent_at
<= kernel_tx <= received_at, all on the client's own wall clock), falling
back to the userspace sent_at.wall sample otherwise. RTT, downstream OWD,
IPDV, and timeout/scheduling semantics are unchanged.

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

@@ -72,7 +72,7 @@ fn compute_one_way_omits_samples_when_client_wall_time_overflows_i64() {
     };
 
     assert_eq!(
-        compute_one_way(&sent_at, &received_at, &ReceiveMeta::default(), &ts),
+        compute_one_way(&sent_at, &received_at, None, &ReceiveMeta::default(), &ts),
         None
     );
 }
@@ -93,7 +93,8 @@ fn compute_one_way_returns_available_direction_samples() {
         ..Default::default()
     };
 
-    let sample = compute_one_way(&sent_at, &received_at, &ReceiveMeta::default(), &ts).unwrap();
+    let sample =
+        compute_one_way(&sent_at, &received_at, None, &ReceiveMeta::default(), &ts).unwrap();
     assert_eq!(
         sample.client_to_server,
         Some(SignedDuration::from_nanos(15_000_000))
@@ -120,7 +121,8 @@ fn compute_one_way_preserves_negative_client_to_server_delay() {
         ..Default::default()
     };
 
-    let sample = compute_one_way(&sent_at, &received_at, &ReceiveMeta::default(), &ts).unwrap();
+    let sample =
+        compute_one_way(&sent_at, &received_at, None, &ReceiveMeta::default(), &ts).unwrap();
 
     assert_eq!(
         sample.client_to_server,
@@ -148,7 +150,8 @@ fn compute_one_way_preserves_negative_server_to_client_delay() {
         ..Default::default()
     };
 
-    let sample = compute_one_way(&sent_at, &received_at, &ReceiveMeta::default(), &ts).unwrap();
+    let sample =
+        compute_one_way(&sent_at, &received_at, None, &ReceiveMeta::default(), &ts).unwrap();
 
     assert_eq!(
         sample.client_to_server,