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/packet.rs

@@ -57,6 +57,15 @@ impl DnsPacket {
         }
     }
 
+    pub fn query(id: u16, domain: &str, qtype: crate::question::QueryType) -> DnsPacket {
+        let mut pkt = DnsPacket::new();
+        pkt.header.id = id;
+        pkt.header.recursion_desired = true;
+        pkt.questions
+            .push(crate::question::DnsQuestion::new(domain.to_string(), qtype));
+        pkt
+    }
+
     pub fn response_from(query: &DnsPacket, rescode: crate::header::ResultCode) -> DnsPacket {
         let mut resp = DnsPacket::new();
         resp.header.id = query.header.id;