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/packet.rs

@@ -68,24 +68,29 @@ impl DnsPacket {
     }
 
     pub fn write(&self, buffer: &mut BytePacketBuffer) -> Result<()> {
+        // Filter out UNKNOWN records (e.g. EDNS OPT) that we can't re-serialize
+        let answers: Vec<_> = self.answers.iter().filter(|r| !r.is_unknown()).collect();
+        let authorities: Vec<_> = self.authorities.iter().filter(|r| !r.is_unknown()).collect();
+        let resources: Vec<_> = self.resources.iter().filter(|r| !r.is_unknown()).collect();
+
         let mut header = self.header.clone();
         header.questions = self.questions.len() as u16;
-        header.answers = self.answers.len() as u16;
-        header.authoritative_entries = self.authorities.len() as u16;
-        header.resource_entries = self.resources.len() as u16;
+        header.answers = answers.len() as u16;
+        header.authoritative_entries = authorities.len() as u16;
+        header.resource_entries = resources.len() as u16;
 
         header.write(buffer)?;
 
         for question in &self.questions {
             question.write(buffer)?;
         }
-        for rec in &self.answers {
+        for rec in answers {
             rec.write(buffer)?;
         }
-        for rec in &self.authorities {
+        for rec in authorities {
             rec.write(buffer)?;
         }
-        for rec in &self.resources {
+        for rec in resources {
             rec.write(buffer)?;
         }