Phase 1.2: SMB3 encryption negotiation + session state

- Add encryption_supported and encryption_cipher to Connection state
- Add encryption_key and encryption_enabled to Session state
- Add EncryptionCapabilities context to NegotiateResponse (SMB 3.1.1)
- Derive encryption_key from session_base_key in session_setup
- Export derive_encryption_key as public method
- Fix Session::new() signature with 8 parameters
- All encryption tests pass (3 passed)

vendor/smb-server/src/proto/crypto/encryption.rs

@@ -224,7 +224,7 @@ impl Smb3Encryption {
         nonce
     }
     
-    fn derive_encryption_key(session_key: &[u8], context: &[u8]) -> [u8; 16] {
+    pub fn derive_encryption_key(session_key: &[u8], context: &[u8]) -> [u8; 16] {
         use sha2::{Sha256, Digest};
         
         let mut hasher = Sha256::new();