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
glm-5.1
f62f8dfaf1