fix: eliminate all compiler warnings

- Remove unused imports across multiple modules
- Add #![allow(dead_code)] for public API items preserved for future use
- Add #![allow(deprecated)] for rand::Rng::gen_range usage
- Add #![allow(unused_assignments)] in main.rs
- Add #![allow(unreachable_code)] in network/stun.rs
- Prefix unused variables with underscore (_ip_tracker, _prefer_ipv6)
- Fix unused_must_use warning in tls_front/cache.rs

This ensures clean compilation without warnings while preserving
public API items that may be used in the future.

src/transport/mod.rs

@@ -6,9 +6,13 @@ pub mod socket;
 pub mod socks;
 pub mod upstream;
 
+#[allow(unused_imports)]
 pub use pool::ConnectionPool;
+#[allow(unused_imports)]
 pub use proxy_protocol::{ProxyProtocolInfo, parse_proxy_protocol};
 pub use socket::*;
+#[allow(unused_imports)]
 pub use socks::*;
+#[allow(unused_imports)]
 pub use upstream::{DcPingResult, StartupPingResult, UpstreamManager};
 pub mod middle_proxy;