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/receive/linux/error_queue.rs

@@ -153,7 +153,14 @@ impl ErrorQueueControlBuffer {
 
 /// Nonblocking drain of a single record from `fd`'s `MSG_ERRQUEUE`.
 ///
-/// Returns `Ok(None)` when the queue is empty (`EAGAIN`/`EWOULDBLOCK`).
+/// Returns `Ok(None)` when the queue is empty (`EAGAIN`/`EWOULDBLOCK`) or
+/// when this one read was interrupted (`EINTR`). TX timestamp capture is
+/// optional best-effort metadata, not a network operation: an interrupted
+/// read here is not a socket/network error and must not become one, so it is
+/// treated as no record obtained on this drain rather than retried in a
+/// loop. The bounded caller in [`super::drain_tx_timestamps`] naturally
+/// revisits the queue on its next call, which is how every other record is
+/// picked up too.
 /// `SOF_TIMESTAMPING_OPT_TSONLY` notifications carry no meaningful payload,
 /// so a zero-length iovec is enough: correlation is by `id`, never by
 /// payload content.
@@ -174,7 +181,7 @@ pub(crate) fn try_recv_error_queue_record(fd: RawFd) -> io::Result<Option<ErrorQ
                 .map_err(|_| io::Error::from(io::ErrorKind::InvalidData))?;
             Ok(Some(classify(cmsgs)))
         }
-        Err(Errno::EWOULDBLOCK) => Ok(None),
+        Err(Errno::EWOULDBLOCK | Errno::EINTR) => Ok(None),
         Err(errno) => Err(io::Error::from(errno)),
     }
 }