Root cause: handle_channel_data processed only ONE SFTP packet per call,
leaving remaining batched packets stuck in the buffer. Client waited for
READ responses while server waited for more data — deadlock after ~3.1MB.
Fix:
- sftp_handler.rs: fix SSH_FXP_VERSION format (remove uint32 extension_count)
- sftp_handler.rs: fix handle_open error mapping (.ok() → build_status_from_io_error)
- channel.rs: batch-process ALL complete SFTP packets from buffer in loop
- channel.rs: add pending_packets VecDeque for multi-response queuing
- channel.rs: chain WINDOW_ADJUST + SFTP response when window is low
- channel.rs: add adjust_remote_window() for client WINDOW_ADJUST
- server.rs: drain pending_packets after each CHANNEL_DATA handler
Verified: 5MB upload + download with matching MD5
83fb0de78a