fix FORMERR: filter UNKNOWN records and increase buffer to 4096

Root cause: upstream resolvers return EDNS OPT records (type 41) in
the additional section. Our parser reads them as UNKNOWN, but write()
silently skips them — creating a header that claims N additional records
but a body with 0, producing FORMERR on the client side.

Fix: filter out UNKNOWN records before serialization and adjust header
counts to match. Also increase BytePacketBuffer from 512 to 4096 bytes
to handle modern DNS responses with many records.

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

src/record.rs

@@ -43,6 +43,10 @@ pub enum DnsRecord {
 }
 
 impl DnsRecord {
+    pub fn is_unknown(&self) -> bool {
+        matches!(self, DnsRecord::UNKNOWN { .. })
+    }
+
     pub fn ttl(&self) -> u32 {
         match self {
             DnsRecord::A { ttl, .. }