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/util/mod.rs

@@ -3,5 +3,7 @@
 pub mod ip;
 pub mod time;
 
+#[allow(unused_imports)]
 pub use ip::*;
+#[allow(unused_imports)]
 pub use time::*;