Add a proxy protocol

The examples are module/proxy/socks5 and module/proxy/http. A new stream proxy touches:

File	Change
module/proxy/<name>/settings.go (new)	settings, Validate, Warnings
module/proxy/<name>/proxy.go (new)	Proxy type, Info, NewStreamServer, NewStreamClient, NewProxyModule
module/proxy/<name>/server.go, client.go (new)	the two roles
module/proxy/<name>/wire/ (new, optional)	the byte format, kept separate so it can be tested and fuzzed alone
module/proxy/<name>/<name>_test.go (new)	tests over net.Pipe, no core
core/registry/default.go	one line: RegisterStream(<name>.NewProxyModule)

Rules

• Server. AcceptStream(ctx, underlyingStream) reads the request and returns the payload stream and spec.Metadata with the destination, within ctx.
• Client. RequestStream(ctx, underlyingStream, metadata) writes the request onto a stream that already reaches the server, or holds it for the first write.
• Never dial. The stream is handed to you. A protocol that truly needs a second socket is a design question: raise it first.
• Fallback. For a request that is not your protocol, return spec.ErrFallbackRequested before writing anything; core replays it to the inbound's fallback.
• Wrong credentials. Wrap spec.ErrAuthenticationFailed, so core counts them apart from garbage.

Users

If clients log in, implement spec.UserManager (AddUser, RemoveUser, ListUsers) on the server; module/proxy/trojan is the example.

• A user is one element of your users list, decoded with unknown keys refused.
• Return spec.ErrInvalidUser, spec.ErrUserExists or spec.ErrNoSuchUser, with no credential in the message.
• Keep users in an immutable snapshot behind an atomic.Pointer, and compare in constant time.
• Refuse the other role's keys in NewStreamServer and NewStreamClient, so a stray key cannot leave an inbound open.

UDP

A protocol with UDP also fills spec.DatagramOverStreamProxyModule (UDP framed in the stream) or spec.DatagramProxyModule. It stays one module: NewProxyModule returns the combination, such as spec.StreamAndDatagramOverStreamProxyModule, and is registered once with RegisterStreamAndDatagramOverStream.

• Build the server once, so both kinds share one object (sync.Once in trojan).
• Implement IsDatagramOverStream, which only peeks, so core can choose between AcceptStream and AcceptDatagramOverStream.