feat: define Interface trait and InterfaceConfig types (core/interface-trait-definition)

Add Layer 2 interface abstraction per ADR-026:
- Interface trait with accept() and associated Session type
- InterfaceConfig enum with Ssh and RawFraming variants
- SshInterfaceConfig with auth, forwarding, host_key fields
- RawFramingConfig (minimal, no SSH-specific config)
- InterfaceSession trait with recv()/send() producing InterfaceEvent frames
- InterfaceEvent wraps EventEnvelope with optional Identity
- Resolves OQ-IF-01: every session produces EventEnvelope frames
  via InterfaceSession, making Layer 3 interface-agnostic
- Valid (Transport, Interface) pair enumeration with
  TransportKindBase and is_valid_pair validation function
- Module re-exported from lib.rs

crates/alknet-core/src/lib.rs

@@ -56,6 +56,7 @@ pub mod call;
 pub mod client;
 pub mod config;
 pub mod error;
+pub mod interface;
 pub mod server;
 pub mod socks5;
 pub mod transport;
@@ -84,5 +85,10 @@ pub use config::{
     ForwardingPolicy, ForwardingRule, RateLimitConfig, StaticConfig, TargetPattern,
 };
 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,
+};
 pub use server::serve::{ListenerConfig, ServeError, ServeOptions, ServeTransportMode, Server};
 pub use transport::{Transport, TransportAcceptor, TransportInfo, TransportKind};