Implement SSH Phase 13: Port forwarding foundation

- Add port_forward.rs module (285 lines)
- Define PortForwardType enum (Local/Remote/Dynamic)
- Implement PortForwardManager for managing forwards
- Handle SSH_MSG_GLOBAL_REQUEST for tcpip-forward
- Handle direct-tcpip and forwarded-tcpip channel types
- Support Local port forwarding (-L) foundation
- Support Remote port forwarding (-R) foundation
- Ready for integration with channel.rs

markbase-core/src/ssh_server/mod.rs

@@ -14,6 +14,7 @@ pub mod channel;
 pub mod sftp_handler;
 pub mod scp_handler;
 pub mod rsync_handler;
+pub mod port_forward;  // Phase 13: 端口转发模块
 
 pub use server::SshServer;
 pub use packet::{SshPacket, PacketType};