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/middle_proxy/pool.rs

@@ -36,6 +36,7 @@ pub struct MeWriter {
     pub allow_drain_fallback: Arc<AtomicBool>,
 }
 
+#[allow(dead_code)]
 pub struct MePool {
     pub(super) registry: Arc<ConnRegistry>,
     pub(super) writers: Arc<RwLock<Vec<MeWriter>>>,
@@ -992,6 +993,7 @@ impl MePool {
 
 }
 
+#[allow(dead_code)]
 fn hex_dump(data: &[u8]) -> String {
     const MAX: usize = 64;
     let mut out = String::with_capacity(data.len() * 2 + 3);