fix(io): retry interrupted receives

Real signal delivery on Linux (observed via x86_64-musl workspace
testing) can interrupt blocking and nonblocking recv/recvmsg calls
with EINTR. Several production and test receive paths treated that as
a fatal error rather than a transient interruption:

- Client::open's blocking receive loop and Client::recv_once/
  recv_available's blocking receive returned ClientError::Socket on
  Interrupted instead of retrying within their existing timeout
  budget.
- AsyncClient::poll_open and poll_recv's nonblocking recvmsg path
  (Linux ancillary) failed the same way instead of retrying through
  the existing readiness/work-budget machinery.
- The server's ancillary receive loop could terminate the listener on
  an interrupted nonblocking recvmsg.
- The client's MSG_ERRQUEUE TX-timestamp drain turned an interrupted
  read of optional metadata into a fatal ClientError::Socket.
- Fake-server test helpers panicked or misclassified EINTR as a
  timeout instead of retrying it.

Interrupted is now retried transparently everywhere a receive can
observe it, while preserving each path's existing timing contract:

- Blocking Open reuses its already-absolute per-attempt deadline, so
  interruptions resume within the same attempt without retransmitting
  or extending the deadline.
- recv_once/recv_available compute one absolute deadline per logical
  receive (when a timeout is configured) so repeated interruptions
  cannot extend it past the configured timeout, and restore the
  socket's configured read timeout afterward so a later call in the
  same recv_available budget is unaffected.
- AsyncClient's poll_open already bounds its retry loop with
  OPEN_POLL_WORK_BUDGET; poll_recv gains an equivalent
  RECV_POLL_WORK_BUDGET, since unlike WouldBlock, Interrupted does not
  clear Tokio's readiness state, so an unbounded retry loop was
  otherwise possible under sustained signal delivery.
- The server's receive loop retries Interrupted the same way it
  already retries WouldBlock, yielding to the runtime after a bounded
  number of consecutive interruptions for the same reason.
- The error-queue drain treats an interrupted read as no record
  obtained this drain (deferring to the next natural drain point)
  rather than a socket error, matching its existing EWOULDBLOCK
  handling.

Deterministic regression tests use a small test-only injection hook
(mirroring existing send-injection test hooks) rather than real signal
timing, covering: Open completing despite interruptions with no
retransmit, Open's deadline not extending under repeated
interruptions, recv_once classifying a reply normally despite
interruptions, recv_once's timeout not extending under repeated
interruptions, and the AsyncClient equivalents for open and recv.

No protocol, session, or timeout-classification behavior changes; no
new dependencies; send-side EINTR handling is unchanged and out of
scope.

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

@@ -883,6 +883,53 @@ fn receive_retries_false_readiness_and_shares_packet_classification() {
     assert_eq!(server.finish().len(), 2);
 }
 
+#[test]
+fn receive_retries_interrupted_syscall_and_shares_packet_classification() {
+    let server = start_server(move |socket, tx| {
+        let (params, peer) = open_session(&socket, &tx, None);
+        let (probe_packet, _) = recv_packet(&socket, &tx);
+        let sequence = echo_request_sequence(&probe_packet, None);
+        socket
+            .send_to(
+                &echo_reply(&params, sequence, TOKEN, flags::FLAG_REPLY, None),
+                peer,
+            )
+            .unwrap();
+    });
+
+    runtime().block_on(async {
+        let mut client = opened_client(server.addr, 0).await;
+        client.send_probe().await.unwrap();
+        client.test_hooks.inject_recv_interrupted.set(2);
+
+        assert!(matches!(
+            client.recv().await.unwrap().as_slice(),
+            [ClientEvent::EchoReply { seq: 0, .. }]
+        ));
+    });
+    assert_eq!(server.finish().len(), 2);
+}
+
+#[test]
+fn open_retries_interrupted_syscall_within_the_same_attempt() {
+    let server = start_open_server(None, 0);
+    runtime().block_on(async {
+        let mut client = AsyncClient::connect(config(server.addr, None, 0))
+            .await
+            .unwrap();
+        client.test_hooks.inject_recv_interrupted.set(3);
+
+        assert!(matches!(
+            client.open().await,
+            Ok(OpenOutcome::Started { .. })
+        ));
+        // Only one open request reached the server: interruptions were
+        // absorbed inside the single open attempt, with no retransmit.
+        assert_eq!(client.test_hooks.send_attempts.get(), 1);
+    });
+    server.finish();
+}
+
 #[test]
 fn peer_close_dscp_failure_preserves_events_and_logical_ownership() {
     let server = start_peer_close_server();