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/stream/crypto_stream.rs

@@ -18,6 +18,8 @@
 //!   is either written to upstream or stored in our pending buffer
 //! - when upstream is pending -> ciphertext is buffered/bounded and backpressure is applied
 //!
+
+#![allow(dead_code)]
 //! =======================
 //! Writer state machine
 //! =======================
@@ -55,7 +57,7 @@ use std::io::{self, ErrorKind, Result};
 use std::pin::Pin;
 use std::task::{Context, Poll};
 use tokio::io::{AsyncRead, AsyncWrite, ReadBuf};
-use tracing::{debug, trace, warn};
+use tracing::{debug, trace};
 
 use crate::crypto::AesCtr;
 use super::state::{StreamState, YieldBuffer};