fix: advisory + exit(1) when port 53 is already in use (#45)

Detect AddrInUse on bind, print a human-readable diagnostic explaining
systemd-resolved / Dnscache as the likely cause and offer two concrete
fixes (sudo numa install, or bind_addr on a non-privileged port), then
exit(1) instead of surfacing a raw OS error.

Adds tests/docker/smoke-port53.sh: end-to-end Docker test that
pre-binds port 53 with a Python UDP socket and asserts the advisory +
exit code.

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

src/main.rs

@@ -231,8 +231,23 @@ async fn main() -> numa::Result<()> {
         None
     };
 
+    let socket = match UdpSocket::bind(&config.server.bind_addr).await {
+        Ok(s) => s,
+        Err(e)
+            if e.kind() == std::io::ErrorKind::AddrInUse
+                && numa::system_dns::is_port_53(&config.server.bind_addr) =>
+        {
+            eprint!(
+                "{}",
+                numa::system_dns::port53_conflict_advisory(&config.server.bind_addr)
+            );
+            std::process::exit(1);
+        }
+        Err(e) => return Err(e.into()),
+    };
+
     let ctx = Arc::new(ServerCtx {
-        socket: UdpSocket::bind(&config.server.bind_addr).await?,
+        socket,
         zone_map: build_zone_map(&config.zones)?,
         cache: RwLock::new(DnsCache::new(
             config.cache.max_entries,