Implement full AVP protocol

- Core types (Secret, Session, Workspace, Backend, etc.)
- Error types with thiserror
- AVPClient with all 7 operations (DISCOVER, AUTHENTICATE, STORE, RETRIEVE, DELETE, LIST, ROTATE)
- MemoryBackend for testing
- FileBackend with AES-256-GCM encryption
- Unit tests included
- Requires: chrono, serde, aes-gcm, pbkdf2