Implement ServerHandler with auth delegation and channel dispatch

Convert server.rs to directory module (server/mod.rs + server/handler.rs).
ServerHandler implements russh::server::Handler with:
- auth_publickey() delegating to ServerAuthConfig with structured logging
- channel_open_direct_tcpip() routing wraith-* prefix to internal handler,
  stub for regular TCP proxy
- ProxyConfig/ProxyMode types for outbound proxy configuration
- Unit tests for auth delegation, reserved destination routing, and
  unknown channel type rejection

crates/wraith-core/src/server/mod.rs

@@ -0,0 +1,3 @@
+pub mod handler;
+
+pub use handler::{ProxyConfig, ProxyMode, ServerHandler};