refactor: deduplicate query builders, record extraction, sinkhole records (#22)

- Add DnsPacket::query(id, domain, qtype) constructor; replace mock_query,
  make_query, and 4 inline constructions across ctx/forward/recursive/api
- Add record_to_addr() in recursive.rs; replace 4 identical A/AAAA match
  blocks with filter_map one-liners
- Add sinkhole_record() in ctx.rs; consolidate localhost and blocklist
  A/AAAA branching into single calls
- Remove now-unused DnsQuestion imports

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/api.rs

@@ -410,14 +410,8 @@ async fn forward_query_for_diagnose(
     timeout: std::time::Duration,
 ) -> (bool, String) {
     use crate::packet::DnsPacket;
-    use crate::question::DnsQuestion;
 
-    let mut query = DnsPacket::new();
-    query.header.id = 0xBEEF;
-    query.header.recursion_desired = true;
-    query
-        .questions
-        .push(DnsQuestion::new(domain.to_string(), QueryType::A));
+    let query = DnsPacket::query(0xBEEF, domain, QueryType::A);
 
     match forward_query(&query, upstream, timeout).await {
         Ok(resp) => (