Add build_kexdh_reply logging to verify server_public_key

验证server_public_key一致性：
- build_kexdh_reply输入：[156, 109, 160, 110, ...]
- crypto.rs中的值：[156, 109, 160, 110, ...]
- 完全一致 ✓

Packet capture验证：
- Client public key：d9a035145879e1c6...（与server logs完全匹配）
- Server public key：9c6da06e74b7e55c...（与server logs完全匹配）

关键发现：
- 所有public keys完全匹配
- Client计算的shared_secret ≠ Server（仍需调查）

下一步：
继续调查shared secret encoding差异

markbase-core/src/ssh_server/kex_exchange.rs

@@ -125,6 +125,9 @@ impl KexExchangeHandler {
         host_key_blob: &[u8],
         server_public_key: &[u8],
     ) -> Result<SshPacket> {
+        info!("=== Building SSH_MSG_KEXDH_REPLY ===");
+        info!("Input server_public_key: {:?}", server_public_key);
+        
         let mut payload = Vec::new();
         
         payload.write_u8(PacketType::SSH_MSG_KEXDH_REPLY as u8)?;
@@ -132,6 +135,7 @@ impl KexExchangeHandler {
         payload.write_u32::<BigEndian>(host_key_blob.len() as u32)?;
         payload.write_all(host_key_blob)?;
         
+        info!("Writing server_public_key to payload (32 bytes)");
         payload.write_u32::<BigEndian>(32)?;
         payload.write_all(server_public_key)?;
         
@@ -139,6 +143,7 @@ impl KexExchangeHandler {
         payload.write_u32::<BigEndian>(signature.len() as u32)?;
         payload.write_all(&signature)?;
         
+        info!("SSH_MSG_KEXDH_REPLY payload built successfully");
         Ok(SshPacket::new(payload))
     }