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/tests/support/mod.rs

@@ -4,6 +4,7 @@ mod in_tree_server;
 mod real_irtt;
 
 use std::{
+    io::ErrorKind,
     net::{SocketAddr, UdpSocket},
     process::Command,
     sync::mpsc,
@@ -447,17 +448,31 @@ where
     FakeServer { addr, rx, done }
 }
 
+/// A fake server's `recv_from`, retrying an interrupted syscall
+/// (`EINTR`/`ErrorKind::Interrupted`) instead of panicking on it.
 fn recv_request(socket: &UdpSocket) -> (Vec<u8>, SocketAddr) {
     let mut buf = [0_u8; 8192];
-    let (size, peer) = socket.recv_from(&mut buf).unwrap();
-    (buf[..size].to_vec(), peer)
+    loop {
+        match socket.recv_from(&mut buf) {
+            Ok((size, peer)) => return (buf[..size].to_vec(), peer),
+            Err(err) if err.kind() == ErrorKind::Interrupted => continue,
+            Err(err) => panic!("fake server recv_from failed: {err}"),
+        }
+    }
 }
 
+/// Like [`recv_request`], but any non-interrupting error (including a
+/// configured read timeout) means "nothing more to receive" rather than a
+/// failure. An interrupted syscall carries no such information and is
+/// retried instead of being misclassified as quiet.
 fn recv_request_timeout(socket: &UdpSocket) -> Option<Vec<u8>> {
     let mut buf = [0_u8; 8192];
-    match socket.recv_from(&mut buf) {
-        Ok((size, _)) => Some(buf[..size].to_vec()),
-        Err(_) => None,
+    loop {
+        match socket.recv_from(&mut buf) {
+            Ok((size, _)) => return Some(buf[..size].to_vec()),
+            Err(err) if err.kind() == ErrorKind::Interrupted => continue,
+            Err(_) => return None,
+        }
     }
 }