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/async_client.rs

@@ -528,15 +528,20 @@ impl AsyncClient {
 
             let expected_bytes = prepared.bytes.len();
             let scheduled_at = schedule_commit.scheduled_at;
+            // Private pre-send anchor: finalizes all fallible commit work
+            // (timeout deadline arithmetic, kernel TX lower bound) before
+            // the send attempt. A WouldBlock retry samples a fresh one on
+            // the next loop iteration; only the successful attempt's anchor
+            // is retained (via `machine_commit`/`commit_probe_sent` below).
             #[cfg(not(test))]
-            let sent_at = ClientTimestamp::now();
+            let send_anchor = ClientTimestamp::now();
             #[cfg(test)]
-            let sent_at = test_timestamps
-                .map(|timestamps| timestamps.sent_at)
+            let send_anchor = test_timestamps
+                .map(|timestamps| timestamps.send_anchor)
                 .unwrap_or_else(ClientTimestamp::now);
             let machine_commit = match self
                 .machine
-                .finalize_probe_commit(machine_preflight, sent_at)
+                .finalize_probe_commit(machine_preflight, send_anchor)
             {
                 Ok(commit) => commit,
                 Err(error) => return Poll::Ready(Err(error)),
@@ -553,6 +558,9 @@ impl AsyncClient {
             let send_result = self.socket.try_send(&prepared.bytes);
             let bytes = match send_result {
                 Ok(bytes) => bytes,
+                // No sent_at measurement, no machine commit, no schedule
+                // commit: the transaction is fully discarded and retried
+                // with a fresh send_anchor on the next loop iteration.
                 Err(error) if error.kind() == io::ErrorKind::WouldBlock => continue,
                 Err(error) => return Poll::Ready(Err(ClientError::Socket(error))),
             };
@@ -563,7 +571,18 @@ impl AsyncClient {
             let send_finished_at = test_timestamps
                 .map(|timestamps| timestamps.send_finished_at)
                 .unwrap_or_else(Instant::now);
-            let sent = self.machine.commit_probe_sent(machine_commit, bytes);
+            // Public post-send measurement, captured only after the
+            // successful send above. The subsequent machine/schedule commit
+            // is infallible.
+            #[cfg(not(test))]
+            let sent_at = ClientTimestamp::now();
+            #[cfg(test)]
+            let sent_at = test_timestamps
+                .map(|timestamps| timestamps.sent_at)
+                .unwrap_or_else(ClientTimestamp::now);
+            let sent = self
+                .machine
+                .commit_probe_sent(machine_commit, sent_at, bytes);
             schedule.commit(schedule_commit);
             self.prepared_probe = None;