This commit introduces initial, unstable support for Unix domain sockets
(UDS) on Windows, behind the `windows_unix_domain_sockets` feature gate
Added types:
- `std::os::windows::net::SocketAddr`: represents a UDS address with support
for pathname addresses (abstract and unnamed are parsed but not yet fully
supported).
- `std::os::windows::net::UnixListener`: server-side UDS listener.
- `std::os::windows::net::UnixStream`: client/server stream for UDS.
Key features:
- Binding and connecting using filesystem paths.
- Basic I/O via `Read`/`Write`.
- Address querying (`local_addr`, `peer_addr`).
- Non-blocking mode, timeouts, and socket duplication.
- Includes basic test coverage for smoke, echo, path length, and bind reuse.
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