feat(client): implement port forwarding — local (-L) and remote (-R) forwards

- PortForwardSpec parses -L/-R spec strings: bind_addr:bind_port:target_host:target_port
- LocalForwarder binds TcpListener, accepts connections, opens SSH direct-tcpip channel, proxies bidirectionally
- RemoteForwarder sends tcpip_forward request, handles forwarded-tcpip channel opens, connects local target, proxies bidirectionally
- Both forwarders run concurrently with SOCKS5 server via Arc<Mutex<Handle>>
- Connection errors close individual channels without affecting other forwards or SSH session
- ForwardError type added with display and source chaining tests
- Unit tests: spec parsing, local forward bind/accept, remote forward proxy bidirectional

crates/wraith-core/src/lib.rs

@@ -8,5 +8,5 @@ pub mod error;
 #[cfg(feature = "testutil")]
 pub mod testutil;
 
-pub use error::{AuthError, ChannelError, ConfigError, TransportError};
+pub use error::{AuthError, ChannelError, ConfigError, ForwardError, TransportError};
 pub use transport::{Transport, TransportAcceptor, TransportInfo, TransportKind};