loopd: read on a socket whose peer closed, instead of failing on macOS

macOS refuses every socket option with EINVAL once the peer has closed
(XNU sosetoptlock, bsd/kern/uipc_socket.c), even with unread data still
buffered. loopd set a read timeout before each read, so on macOS every
frame or response that arrived just before the peer closed was lost:
BrokerPort reported the broker unavailable with "os error 22", and the
llama client failed the same way. Twelve loopd test binaries failed on
macOS; Linux never refuses, so the gate on Talos did not see it.

Both places now go through socket::set_read_timeout, which on Apple
targets takes that one refusal as success: a socket shut in both
directions returns its data or the end at once and cannot block. A
zero timeout is still an error.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/loopd/src/socket.rs

@@ -0,0 +1,21 @@
+//! Unix socket options that behave the same on Linux and macOS.
+
+use std::io;
+use std::os::unix::net::UnixStream;
+use std::time::Duration;
+
+/// `EINVAL` on Darwin (`sys/errno.h`); std does not name it and a crate for one number is too much.
+#[cfg(target_vendor = "apple")]
+const EINVAL: i32 = 22;
+
+/// Set the read timeout for the next read. macOS refuses every socket option with `EINVAL` once
+/// the peer has closed (XNU `sosetoptlock`: both `SS_CANTRCVMORE` and `SS_CANTSENDMORE` set), even
+/// with unread data still buffered. A read on such a socket returns that data or the end at once
+/// and cannot block, so there is no timeout left to set and the refusal is not an error.
+pub fn set_read_timeout(stream: &UnixStream, timeout: Duration) -> io::Result<()> {
+    match stream.set_read_timeout(Some(timeout)) {
+        #[cfg(target_vendor = "apple")]
+        Err(e) if e.raw_os_error() == Some(EINVAL) => Ok(()),
+        other => other,
+    }
+}