- Add socket_compat.h/c with platform abstraction for POSIX/Windows
- socket_t typedef: int on POSIX, SOCKET on Windows
- Add socket_platform_init/cleanup for WSAStartup on Windows
- Add socket operations: create_udp, sendto, recvfrom, set_nonblocking
- Add socket options: set_buffers, set_reuseaddr, set_mark (Linux), bind_to_device (Linux)
- Update u_async: add socket_t support with uasync_add_socket_t/remove_socket_t
- Update ETCP: use socket_t and socket_compat functions
- Add DEBUG_CATEGORY_SOCKET for socket debugging
- All 22 tests pass on Linux
- MSYS2 UCRT64 compatible
Evgeny