Add sftp_input_buffer for SFTP packet accumulation (Critical fix preparation)

**Problem Diagnosed**:
- SFTP packet incomplete errors: expected 32797 bytes, have 8192
- SCP/rsync large files create empty files (0B)
- SSH splits large SFTP packets into multiple SSH_MSG_CHANNEL_DATA

**Root Cause**:
- No packet accumulation for SFTP/SCP subsystems
- Each SSH_MSG_CHANNEL_DATA processed independently
- Large SFTP packets (>8KB) split by SSH layer

**Fix Added**:
- sftp_input_buffer: Vec<u8> field in Channel struct
- Initialization in all 3 Channel creation locations
- Ready for packet accumulation logic implementation

**Testing Results**:
- SSH command execution: SUCCESS ✅
- Small file rsync (<=1MB): SUCCESS ✅ (MD5 verified)
- Large file rsync (>=2MB): FAILED ❌
- SFTP/SCP: Packet parsing errors ❌

**Phase 14.2 Status**:
- Poll mechanism: 100% complete ✅
- SFTP/SCP subsystem: Needs packet accumulation fix
- Next: Implement accumulation logic in handle_channel_data()

**Progress**: SSH 95% complete, Critical fix identified

markbase-core/src/ssh_server/channel.rs

@@ -123,6 +123,7 @@ impl ChannelManager {
             scp_handler: None,
             rsync_handler: None,
             exec_process: None,  // Phase 14: 交互式exec
+            sftp_input_buffer: Vec::new(),  // ⭐⭐⭐⭐⭐ Phase 14.2修复：SFTP packet累积
             direct_tcpip: None,
             forwarded_tcpip: None,
         };
@@ -178,6 +179,7 @@ impl ChannelManager {
             scp_handler: None,
             rsync_handler: None,
             exec_process: None,  // Phase 14: 交互式exec
+            sftp_input_buffer: Vec::new(),  // ⭐⭐⭐⭐⭐ Phase 14.2修复
             direct_tcpip: Some(direct_tcpip),
             forwarded_tcpip: None,
         };
@@ -221,6 +223,7 @@ impl ChannelManager {
             scp_handler: None,
             rsync_handler: None,
             exec_process: None,  // Phase 14: 交互式exec
+            sftp_input_buffer: Vec::new(),  // ⭐⭐⭐⭐⭐ Phase 14.2修复
             direct_tcpip: None,
             forwarded_tcpip: Some(forwarded_tcpip),
         };
@@ -1290,6 +1293,8 @@ struct Channel {
     scp_handler: Option<ScpHandler>,  // Phase 8: SCP处理器
     rsync_handler: Option<RsyncHandler>,  // Phase 8: rsync处理器
     exec_process: Option<ExecProcess>,  // Phase 14: 交互式exec进程
+    // ⭐⭐⭐⭐⭐ Critical修复：SFTP packet累积buffer
+    sftp_input_buffer: Vec<u8>,  // Phase 14.2修复：累积不完整的SFTP packets
     // Phase 13.3: 端口转发相关字段
     direct_tcpip: Option<DirectTcpipChannel>,  // direct-tcpip channel（Remote forwarding）
     forwarded_tcpip: Option<ForwardedTcpipChannel>,  // forwarded-tcpip channel（Local forwarding）