feat(net)!: remove owned halves

[Berrysoft]

We don't use a separate type for File, named pipe, or AsyncFd. It's meaningless to assign a new type now. Let's just make the half types itself.

[Copilot]

Pull request overview

This PR removes the dedicated “owned split half” wrapper types from compio-net and instead represents owned halves as the stream type itself (e.g., TcpStream, UnixStream), aligning with the repo’s general approach for ref-counted I/O handles.

Changes:

• Change TcpStream::into_split / UnixStream::into_split to return (Self, Self) via clone instead of OwnedReadHalf/OwnedWriteHalf.
• Update Splittable implementations for TcpStream and UnixStream to use Self as both ReadHalf and WriteHalf.
• Remove the reunite-focused test and add Deref support for borrowed ReadHalf/WriteHalf wrappers.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
compio-net/tests/split.rs	Removes the reunite/unsplit test that depended on the deleted owned-half types.
compio-net/src/unix.rs	Switches UnixStream::into_split and Splittable owned halves to (UnixStream, UnixStream) via cloning.
compio-net/src/tcp.rs	Switches TcpStream::into_split and Splittable owned halves to (TcpStream, TcpStream) via cloning.
compio-net/src/split.rs	Deletes owned-half wrappers and reunite error; adds Deref for borrowed ReadHalf/WriteHalf.

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[AsakuraMizu]

Should we also update the document of Splittable?

[Berrysoft]

The docs of Splittable says,

TcpStream, UnixStream and references to them in compio::net implement this trait without any lock thanks to the underlying sockets’ duplex nature.

I think it's still valid.