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)

Cargo.lock

@@ -963,6 +963,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "78c8292055d1c1df0cce5d180393dc8cce0abec0a7102adb6c7b1eef6016d60a"
 dependencies = [
  "generic-array 0.14.7",
+ "rand_core 0.6.4",
  "typenum",
 ]
 
@@ -2669,6 +2670,7 @@ version = "0.2.0"
 dependencies = [
  "adler",
  "aes 0.8.4",
+ "aes-gcm 0.10.3",
  "anyhow",
  "axum",
  "axum-extra",