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/forward.rs

@@ -0,0 +1,27 @@
+use std::net::SocketAddr;
+use std::time::Duration;
+
+use tokio::net::UdpSocket;
+use tokio::time::timeout;
+
+use crate::buffer::BytePacketBuffer;
+use crate::packet::DnsPacket;
+use crate::Result;
+
+pub async fn forward_query(
+    query: &DnsPacket,
+    upstream: SocketAddr,
+    timeout_duration: Duration,
+) -> Result<DnsPacket> {
+    let socket = UdpSocket::bind("0.0.0.0:0").await?;
+
+    let mut send_buffer = BytePacketBuffer::new();
+    query.write(&mut send_buffer)?;
+
+    socket.send_to(send_buffer.filled(), upstream).await?;
+
+    let mut recv_buffer = BytePacketBuffer::new();
+    timeout(timeout_duration, socket.recv_from(&mut recv_buffer.buf)).await??;
+
+    DnsPacket::from_buffer(&mut recv_buffer)
+}