- 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
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