Phase 4: Implement SSH packet size limit (maxpack - 1024)

- Add maxpack field to SftpHandler structure
- Modify SftpHandler::new() to accept maxpack parameter
- Limit SSH_FXP_READ data size to maxpack - 1024 bytes (OpenSSH style)
- Get maxpack from Channel.remote_maxpacket

Changes:
- sftp_handler.rs: SftpHandler struct + new() + handle_read()
- channel.rs: Pass remote_maxpacket to SftpHandler::new()

Reference: OpenSSH sftp-server.c: process_read()
- Limit: maxpacket - 1024 bytes
- Prevent packet size violation

Test status: 5MB upload still incomplete (2.0MB)
- Issue may require additional debugging
- Upload direction may also need maxpack limit

markbase-core/src/ssh_server/sftp_handler.rs

@@ -256,15 +256,19 @@ pub struct SftpHandler {
     root_dir: PathBuf,
     next_handle_id: u32,
     handles: std::collections::HashMap<u32, SftpHandle>,
+    // ⭐⭐⭐⭐⭐ Phase 4: 添加 client maxpack 限制（参考OpenSSH sftp-server.c）
+    maxpacket: u32,  // 来自 SSH_MSG_CHANNEL_OPEN_CONFIRMATION 的 maximum_packet_size
 }
 
 impl SftpHandler {
-    pub fn new(root_dir: PathBuf) -> Self {
+    // ⭐⭐⭐⭐⭐ Phase 4: 修改 new() 方法，接受 maxpack 参数
+    pub fn new(root_dir: PathBuf, maxpacket: u32) -> Self {
         let canonical_root = root_dir.canonicalize().unwrap_or(root_dir);
         Self {
             root_dir: canonical_root,
             next_handle_id: 0,
             handles: std::collections::HashMap::new(),
+            maxpacket,  // ⭐⭐⭐⭐⭐ Phase 4: client maxpack 限制
         }
     }
 
@@ -420,7 +424,15 @@ impl SftpHandler {
             if let Some(ref mut file) = handle.file {
                 file.seek(SeekFrom::Start(offset))?;
                 
-                let mut buffer = vec![0u8; length as usize];
+                // ⭐⭐⭐⭐⭐ Phase 4: 限制数据大小，不超过 maxpacket - 1024（参考OpenSSH sftp-server.c）
+                // OpenSSH sftp-server.c: process_read() 中限制数据大小为 maxpacket - 1024 bytes
+                // SSH packet overhead: ~1024 bytes (SSH header + SSH_FXP_DATA header)
+                let max_data_size = self.maxpacket - 1024;
+                let actual_length = std::cmp::min(length, max_data_size);
+                
+                info!("SSH_FXP_READ limited: requested={}, actual={}", length, actual_length);
+                
+                let mut buffer = vec![0u8; actual_length as usize];
                 match file.read(&mut buffer) {
                     Ok(0) => {
                         self.build_status_response(id, SftpStatus::SSH_FX_EOF, "End of file")