feat(local): close POSIX/Windows gaps for local stream sockets

- Add Windows make_local_stream_pair() via temp-listener emulation
  of socketpair(), enabling socket-pair tests on IOCP
- Implement assign_socket() in win_local_stream_service so raw
  SOCKET handles can be registered with the IOCP port
- Replace ConnectEx/AcceptEx with blocking connect()/accept() on
  worker threads for AF_UNIX — the IOCP extension functions are
  not reliable for AF_UNIX on all Windows versions
- Enable local_stream_socket tests on Windows (construction, open,
  move, connect/accept, read/write, available, release)
- Add portable temp_socket_dir helper using std::filesystem for
  temp paths across platforms
- Guard local datagram code (SOCK_DGRAM) as POSIX-only at compile
  time — Windows does not support AF_UNIX SOCK_DGRAM
- Remove dead IOCP datagram implementation files
  (win_local_dgram_service.hpp, win_local_dgram_socket.hpp)
- Document Windows limitation on local_datagram_socket and
  local_datagram headers

Author: mvandeberg

include/boost/corosio.hpp

@@ -35,9 +35,15 @@
 #include <boost/corosio/local_stream.hpp>
 #include <boost/corosio/local_stream_socket.hpp>
 #include <boost/corosio/local_stream_acceptor.hpp>
+#include <boost/corosio/local_socket_pair.hpp>
+
+// local_datagram.hpp and local_datagram_socket.hpp are POSIX-only;
+// Windows does not support AF_UNIX datagram sockets (SOCK_DGRAM).
+#include <boost/corosio/detail/platform.hpp>
+#if BOOST_COROSIO_POSIX
 #include <boost/corosio/local_datagram.hpp>
 #include <boost/corosio/local_datagram_socket.hpp>
-#include <boost/corosio/local_socket_pair.hpp>
+#endif
 
 #include <boost/corosio/tls_context.hpp>
 #include <boost/corosio/openssl_stream.hpp>

include/boost/corosio/backend.hpp

@@ -214,8 +214,6 @@ class win_local_stream_socket;
 class win_local_stream_service;
 class win_local_stream_acceptor;
 class win_local_stream_acceptor_service;
-class win_local_dgram_socket;
-class win_local_dgram_service;
 
 class win_signal;
 class win_signals;
@@ -251,8 +249,6 @@ struct iocp_t
     using local_stream_service_type          = detail::win_local_stream_service;
     using local_stream_acceptor_type         = detail::win_local_stream_acceptor;
     using local_stream_acceptor_service_type = detail::win_local_stream_acceptor_service;
-    using local_datagram_socket_type         = detail::win_local_dgram_socket;
-    using local_datagram_service_type        = detail::win_local_dgram_service;
     /// @}
 
     using signal_type           = detail::win_signal;

include/boost/corosio/detail/local_datagram_service.hpp

@@ -11,6 +11,10 @@
 #define BOOST_COROSIO_DETAIL_LOCAL_DATAGRAM_SERVICE_HPP
 
 #include <boost/corosio/detail/config.hpp>
+#include <boost/corosio/detail/platform.hpp>
+
+#if BOOST_COROSIO_POSIX
+
 #include <boost/corosio/local_datagram_socket.hpp>
 #include <boost/corosio/local_endpoint.hpp>
 #include <boost/capy/ex/execution_context.hpp>
@@ -90,4 +94,6 @@ class BOOST_COROSIO_DECL local_datagram_service
 
 } // namespace boost::corosio::detail
 
+#endif // BOOST_COROSIO_POSIX
+
 #endif // BOOST_COROSIO_DETAIL_LOCAL_DATAGRAM_SERVICE_HPP

include/boost/corosio/local_datagram.hpp

@@ -11,6 +11,9 @@
 #define BOOST_COROSIO_LOCAL_DATAGRAM_HPP
 
 #include <boost/corosio/detail/config.hpp>
+#include <boost/corosio/detail/platform.hpp>
+
+#if BOOST_COROSIO_POSIX
 
 namespace boost::corosio {
 
@@ -25,6 +28,9 @@ class local_datagram_socket;
     in the compiled library to avoid exposing platform socket
     headers.
 
+    @note Not available on Windows. Windows does not support
+        AF_UNIX datagram sockets (SOCK_DGRAM).
+
     @see local_datagram_socket
 */
 class BOOST_COROSIO_DECL local_datagram
@@ -45,4 +51,6 @@ class BOOST_COROSIO_DECL local_datagram
 
 } // namespace boost::corosio
 
+#endif // BOOST_COROSIO_POSIX
+
 #endif // BOOST_COROSIO_LOCAL_DATAGRAM_HPP

include/boost/corosio/local_datagram_socket.hpp

@@ -12,6 +12,9 @@
 
 #include <boost/corosio/detail/config.hpp>
 #include <boost/corosio/detail/platform.hpp>
+
+#if BOOST_COROSIO_POSIX
+
 #include <boost/corosio/detail/except.hpp>
 #include <boost/corosio/detail/native_handle.hpp>
 #include <boost/corosio/detail/op_base.hpp>
@@ -56,6 +59,10 @@ namespace boost::corosio {
         kernel filters incoming datagrams to those from the
         connected peer.
 
+    @note Not available on Windows. Windows does not support
+        AF_UNIX datagram sockets (SOCK_DGRAM). Attempting to
+        open this socket on Windows will fail.
+
     @par Cancellation
     All asynchronous operations support cancellation through
     `std::stop_token` via the affine protocol, or explicitly
@@ -871,4 +878,6 @@ class BOOST_COROSIO_DECL local_datagram_socket : public io_object
 
 } // namespace boost::corosio
 
+#endif // BOOST_COROSIO_POSIX
+
 #endif // BOOST_COROSIO_LOCAL_DATAGRAM_SOCKET_HPP

include/boost/corosio/local_socket_pair.hpp

@@ -12,11 +12,11 @@
 
 #include <boost/corosio/detail/config.hpp>
 #include <boost/corosio/detail/platform.hpp>
+#include <boost/corosio/local_stream_socket.hpp>
 
 #if BOOST_COROSIO_POSIX
-
-#include <boost/corosio/local_stream_socket.hpp>
 #include <boost/corosio/local_datagram_socket.hpp>
+#endif
 
 #include <utility>
 
@@ -26,9 +26,10 @@ class io_context;
 
 /** Create a connected pair of local stream sockets.
 
-    Uses socketpair(AF_UNIX, SOCK_STREAM) to create two
-    pre-connected sockets. Data written to one can be read
-    from the other.
+    On POSIX, uses socketpair(AF_UNIX, SOCK_STREAM). On Windows,
+    emulates socketpair by creating a temporary listener, connecting,
+    and accepting. Data written to one socket can be read from the
+    other.
 
     @param ctx The I/O context for the sockets.
 
@@ -39,11 +40,15 @@ class io_context;
 BOOST_COROSIO_DECL std::pair<local_stream_socket, local_stream_socket>
 make_local_stream_pair(io_context& ctx);
 
+#if BOOST_COROSIO_POSIX
 /** Create a connected pair of local datagram sockets.
 
     Uses socketpair(AF_UNIX, SOCK_DGRAM) to create two
     pre-connected sockets.
 
+    @note Not available on Windows. Windows does not support
+        AF_UNIX datagram sockets.
+
     @param ctx The I/O context for the sockets.
 
     @return A pair of connected local datagram sockets.
@@ -52,9 +57,8 @@ make_local_stream_pair(io_context& ctx);
 */
 BOOST_COROSIO_DECL std::pair<local_datagram_socket, local_datagram_socket>
 make_local_datagram_pair(io_context& ctx);
+#endif
 
 } // namespace boost::corosio
 
-#endif // BOOST_COROSIO_POSIX
-
 #endif // BOOST_COROSIO_LOCAL_SOCKET_PAIR_HPP