Complete Phase 15: Window Control + sshbuf zero-copy + SCP support

- Fix Window Control: decrease local_window on SSH_MSG_CHANNEL_DATA (critical fix)
- Implement SSH_MSG_CHANNEL_WINDOW_ADJUST (OpenSSH channels.c style)
- Add sshbuf.rs: zero-copy buffer management (339 lines, OpenSSH sshbuf.c reference)
- Add SCP command detection (scp -t/-f support for legacy protocol)
- Add handle_scp_exec() and handle_interactive_exec() for SCP/rsync
- Verify: rsync 100MB transfer successful, SCP legacy protocol working
- Security: All crypto using RustCrypto authoritative libraries (x25519-dalek, ed25519-dalek, aes, hmac)

markbase-core/src/ssh_server/mod.rs

@@ -14,6 +14,7 @@ pub mod channel;
 pub mod sftp_handler;
 pub mod scp_handler;
 pub mod rsync_handler;
+pub mod sshbuf;                    // Phase 15: SSH Buffer 零拷贝管理（参考OpenSSH sshbuf.c）
 pub mod port_forward;           // Phase 13: 端口转发模块
 pub mod ssh_security_config;    // Phase 13.1: 企业级安全配置
 pub mod port_forward_listener;  // Phase 13.4: 监听线程模块
@@ -24,3 +25,4 @@ pub use server::SshServer;
 pub use packet::{SshPacket, PacketType};
 pub use version::VersionExchange;
 pub use ssh_security_config::SshSecurityConfig;  // Phase 13.1: 导出安全配置
+pub use sshbuf::SshBuf;                            // Phase 15: 导出 SSH Buffer