refactor to async tokio with modular architecture

- Replace synchronous std::net::UdpSocket with tokio async runtime
- Spawn concurrent task per incoming DNS query via tokio::spawn
- Extract monolithic main.rs into modules: buffer, header, question,
  record, packet, config, cache, forward, stats
- Share state across tasks via Arc<ServerCtx> with scoped Mutex locks
- Add TOML config loading, TTL-aware cache, structured logging, stats
- Add CLAUDE.md, README, dns_fun.toml config, and design docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib.rs

@@ -0,0 +1,12 @@
+pub mod buffer;
+pub mod cache;
+pub mod config;
+pub mod forward;
+pub mod header;
+pub mod packet;
+pub mod question;
+pub mod record;
+pub mod stats;
+
+pub type Error = Box<dyn std::error::Error + Send + Sync>;
+pub type Result<T> = std::result::Result<T, Error>;