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>

dns_fun.toml

@@ -0,0 +1,25 @@
+[server]
+bind_addr = "0.0.0.0:53"
+
+[upstream]
+address = "8.8.8.8"
+port = 53
+timeout_ms = 3000
+
+[cache]
+max_entries = 10000
+min_ttl = 60
+max_ttl = 86400
+
+# Example zone records:
+# [[zones]]
+# domain = "dimescu.ro"
+# record_type = "A"
+# value = "3.120.139.105"
+# ttl = 30
+
+# [[zones]]
+# domain = "test.local"
+# record_type = "A"
+# value = "127.0.0.1"
+# ttl = 60