Extract SshInterface from ServerHandler, add RawFramingInterface stub

- SshInterface implements Interface trait with accept() method
- SshSession implements InterfaceSession trait (stub for call protocol events)
- RawFramingInterface is type-only stub (Phase 4+ for DNS, WebTransport)
- TransportKind consolidated into transport module with Display, PartialEq, Eq
- ListenerConfig gains interface_kind field for (Transport, Interface) pairs
- SshInterface wraps existing russh handler logic (SshHandler)
- Auth delegation through IdentityProvider (not embedded in SshInterface)
- Channel routing through session to Layer 3 (forwarding policy)
- Server accept loop uses (Transport, Interface) pairs

Per ADR-026: SSH is Layer 2, not Layer 1. This is the highest-risk Phase 1
task, implementing the Interface trait to separate transport from interface.

crates/alknet-core/src/lib.rs

@@ -87,8 +87,8 @@ pub use config::{
 pub use error::{AuthError, ChannelError, ConfigError, ForwardError, TransportError};
 pub use interface::{
     is_valid_pair, Interface, InterfaceConfig, InterfaceEvent, InterfaceKind, InterfaceSession,
-    RawFramingConfig, SshInterfaceConfig, TransportKindBase, TransportStream,
-    VALID_TRANSPORT_INTERFACE_PAIRS,
+    RawFramingConfig, RawFramingInterface, RawFramingSession, SshInterface, SshInterfaceConfig,
+    SshSession, TransportKindBase, TransportStream, VALID_TRANSPORT_INTERFACE_PAIRS,
 };
 pub use server::serve::{ListenerConfig, ServeError, ServeOptions, ServeTransportMode, Server};
 pub use transport::{Transport, TransportAcceptor, TransportInfo, TransportKind};