fix: resolve compilation warnings with latest stable toolchain

[webees]

Summary

This PR fixes 2 compilation warnings that appear when building with the latest stable Rust toolchain and resolving the latest compatible dependencies (no Cargo.lock is committed in this repo).

Changes

1. Replace deprecated TcpStream::set_linger() (leaf/src/proxy/mod.rs)

- stream.set_linger(Some(Duration::ZERO))?;
+ SockRef::from(&stream).set_linger(Some(Duration::ZERO))?;

Why: Tokio deprecated TcpStream::set_linger() starting from v1.37 because SO_LINGER causes the socket to block the calling thread on drop(). Since the project specifies tokio = { version = "1", ... } without a lockfile, any fresh build today resolves tokio ≥ 1.37 and triggers this warning. The replacement uses socket2::SockRef, which is already a dependency of this project. The runtime behavior is identical.

Reference: tokio-rs/tokio#5765

2. Prefix unused parameter gw with underscore (leaf/src/common/cmd_linux.rs)

- gw: Ipv4Addr,
+ _gw: Ipv4Addr,

Why: The add_interface_ipv4_address() function on Linux uses ip addr add, which does not accept a gateway argument (gateways are set separately via ip route add). The gw parameter exists for API parity with the macOS implementation but is correctly unused on Linux. Prefixing with _ makes this intent explicit and silences the unused_variables warning. This warning has existed since the function was first written but may have been overlooked.

Impact

• Zero behavioral change — both fixes only affect compilation, not runtime logic
• No API breaking changes — function signatures remain identical
• Enables warning-free builds for anyone building the project today