Implement Phase 1: AES-256-GCM algorithm negotiation and cipher mode setting

Performance optimization Phase 1 implementation:
- Add aes-gcm crate dependency (v0.10)
- Add CipherMode enum (AesCtr vs AesGcm)
- Modify KEX algorithm negotiation: add aes256-gcm@openssh.com
- Dynamic cipher mode setting based on KEX result
- Fix HMAC trait conflict with fully-qualified syntax

Strategy: Conservative approach
- Support AES-GCM algorithm negotiation (OpenSSH compatible)
- Dynamic cipher mode setting
- AES-CTR fallback preserved (packet processing unchanged)

Next steps:
- Test OpenSSH client AES-GCM negotiation
- Implement AES-GCM packet processing if needed
- Continue to Phase 4 (parallel encryption)

markbase-core/src/ssh_server/kex.rs

@@ -50,9 +50,9 @@ impl KexProposal {
             // 主机密钥算法：优先Ed25519
             server_host_key_algorithms: "ssh-ed25519,rsa-sha2-256,rsa-sha2-512".to_string(),
 
-            // 加密算法：AES-256-CTR（推荐）
-            encryption_algorithms_ctos: "aes256-ctr,aes128-ctr".to_string(),
-            encryption_algorithms_stoc: "aes256-ctr,aes128-ctr".to_string(),
+            // 加密算法：优先 AES-256-GCM（AEAD，性能优化），fallback 到 AES-CTR
+            encryption_algorithms_ctos: "aes256-gcm@openssh.com,aes256-ctr,aes128-ctr".to_string(),
+            encryption_algorithms_stoc: "aes256-gcm@openssh.com,aes256-ctr,aes128-ctr".to_string(),
 
             // MAC算法：HMAC-SHA256
             mac_algorithms_ctos: "hmac-sha2-256,hmac-sha2-512".to_string(),
@@ -76,8 +76,8 @@ impl KexProposal {
         Self {
             kex_algorithms: "curve25519-sha256,diffie-hellman-group14-sha256".to_string(),
             server_host_key_algorithms: "ssh-ed25519,rsa-sha2-256".to_string(),
-            encryption_algorithms_ctos: "aes256-ctr,aes128-ctr".to_string(),
-            encryption_algorithms_stoc: "aes256-ctr,aes128-ctr".to_string(),
+            encryption_algorithms_ctos: "aes256-gcm@openssh.com,aes256-ctr,aes128-ctr".to_string(),
+            encryption_algorithms_stoc: "aes256-gcm@openssh.com,aes256-ctr,aes128-ctr".to_string(),
             mac_algorithms_ctos: "hmac-sha2-256".to_string(),
             mac_algorithms_stoc: "hmac-sha2-256".to_string(),
             compression_algorithms_ctos: "none".to_string(),