fix(client): timestamp probes after successful send

Moves the client userspace probe sent_at measurement to immediately
after a successful UDP send, improving raw RTT and the userspace
upstream-OWD fallback by excluding the socket send-call interval and
the small amount of local work preceding it.

All fallible probe-commit preflight (timeout deadline checked_add,
capacity/collision checks) still runs before the send, from a private
pre-send anchor (SessionMachine::finalize_probe_commit). The public
sent_at is now supplied afterward to an infallible
SessionMachine::commit_probe_sent, so DurationOverflow still guarantees
nothing was transmitted and a WouldBlock try_send still commits
nothing.

The operational probe timeout deadline stays anchored to the pre-send
timestamp on purpose, not to the new post-send sent_at, preserving
existing timeout/loss timing. Linux kernel TX_SOFTWARE plausibility
keeps its own pre-send wall-clock lower bound
(PendingProbe::tx_not_before_wall) instead of the post-send sent_at,
since a legitimate TX_SOFTWARE timestamp can be generated while the
send syscall is still in flight and would otherwise be wrongly
rejected.

Also flags (without touching production code) that irtt-stats'
ReplySample::from_reply_parts feeds the public sent_at directly into
IpdvSample.client_send_mono/client_send_wall_ns, so the upstream/send
IPDV component naturally shifts along with the sent_at semantics
change; per scope this is reported rather than compensated for.

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

@@ -72,7 +72,14 @@ fn compute_one_way_omits_samples_when_client_wall_time_overflows_i64() {
     };
 
     assert_eq!(
-        compute_one_way(&sent_at, &received_at, None, &ReceiveMeta::default(), &ts),
+        compute_one_way(
+            &sent_at,
+            sent_at.wall,
+            &received_at,
+            None,
+            &ReceiveMeta::default(),
+            &ts
+        ),
         None
     );
 }
@@ -93,8 +100,15 @@ fn compute_one_way_returns_available_direction_samples() {
         ..Default::default()
     };
 
-    let sample =
-        compute_one_way(&sent_at, &received_at, None, &ReceiveMeta::default(), &ts).unwrap();
+    let sample = compute_one_way(
+        &sent_at,
+        sent_at.wall,
+        &received_at,
+        None,
+        &ReceiveMeta::default(),
+        &ts,
+    )
+    .unwrap();
     assert_eq!(
         sample.client_to_server,
         Some(SignedDuration::from_nanos(15_000_000))
@@ -121,8 +135,15 @@ fn compute_one_way_preserves_negative_client_to_server_delay() {
         ..Default::default()
     };
 
-    let sample =
-        compute_one_way(&sent_at, &received_at, None, &ReceiveMeta::default(), &ts).unwrap();
+    let sample = compute_one_way(
+        &sent_at,
+        sent_at.wall,
+        &received_at,
+        None,
+        &ReceiveMeta::default(),
+        &ts,
+    )
+    .unwrap();
 
     assert_eq!(
         sample.client_to_server,
@@ -150,8 +171,15 @@ fn compute_one_way_preserves_negative_server_to_client_delay() {
         ..Default::default()
     };
 
-    let sample =
-        compute_one_way(&sent_at, &received_at, None, &ReceiveMeta::default(), &ts).unwrap();
+    let sample = compute_one_way(
+        &sent_at,
+        sent_at.wall,
+        &received_at,
+        None,
+        &ReceiveMeta::default(),
+        &ts,
+    )
+    .unwrap();
 
     assert_eq!(
         sample.client_to_server,