io_uring event loop

[igorls]

Replace the current epoll-based event loop with io_uring for zero-copy I/O and reduced syscall overhead.

Targets:

• TUN read/write via IORING_OP_READ/IORING_OP_WRITE
• UDP send/recv via IORING_OP_SENDMSG/IORING_OP_RECVMSG
• Registered buffers for zero-copy
• Submission queue polling (IORING_SETUP_SQPOLL) for kernel-side batching