新增: ssh-proxy 文件传输 (并行 SFTP)

基于 russh-sftp 实现多通道并行文件传输，支持 upload/download，
默认 4 通道并行，可通过 -c 参数动态调整并发数。
传输失败时自动清理不完整的远程/本地文件。

ssh-proxy/Cargo.toml

@@ -16,6 +16,7 @@ anyhow = "1"
 clap = { version = "4", features = ["derive"] }
 ureq = "2"  # CLI HTTP client (2.x has simpler API)
 time = { version = "0.3", features = ["formatting", "macros"] }
+russh-sftp = "2.0.8"
 
 [profile.release]
 opt-level = "z"