Support pathname Unix-domain streams on Windows

[LunaStev]

Motivation

The Windows local-address provider always returns -95, and the Windows branches of _unix_socket and unix_remove do the same. Consequently, portable applications using std::net::unix cannot use pathname local stream sockets on Windows amd64 or ARM64.

This is an intentional missing capability in the current source, not a reproduced regression. Microsoft exposes AF_UNIX for Win32 interprocess communication. A focused provider would let applications use the existing stream API for local IPC without substituting a network listener.

Source audit at 4c3372157d8889fa8bbef0a7ccd2e72aab1c5c6c: std/sys/windows/local_addr.wave, std/sys/local_addr.wave, std/net/unix.wave, std/sys/windows/socket.wave.

Scope

After agreeing on the Windows contract, support pathname AF_UNIX/SOCK_STREAM through Winsock and the existing UnixListener/UnixStream operations. Define address encoding, path limits, filesystem entry cleanup and behavior on Windows versions without the capability. Preserve native errors and close resources on failed bind/connect/listen.

Named pipes, Linux abstract addresses, descriptor passing and local datagrams are separate capabilities. Keep unsupported operations explicit, and preserve Linux/macOS/FreeBSD behavior. No libc adapter is required. Related Windows provider tracker: #385.

Difficulty: hard.

Completion criteria

• Native Windows amd64/ARM64 tests bind, connect, accept, exchange bytes, shut down and close pathname streams.
• Test missing/occupied paths, path-length boundaries, agreed non-ASCII path behavior and cleanup after failure.
• Unsupported Windows versions and unsupported local-address modes return the agreed observable error.
• Existing Unix platforms and ordinary Winsock TCP/UDP providers retain their behavior.

Design decision needed: Confirm the minimum Windows version, pathname encoding/limits, cleanup ownership and unsupported-capability behavior before implementing the provider.

Please mention @LunaStev to discuss the design before starting implementation.