fix: add debug logging for DoT SERVFAIL serialization failure, TC-bit TODO

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

src/ctx.rs

@@ -310,6 +310,8 @@ pub async fn resolve_query(
     );
 
     // Serialize response
+    // TODO: TC bit is UDP-specific; DoT connections could carry up to 65535 bytes.
+    // Once BytePacketBuffer supports larger buffers, skip truncation for TCP/TLS.
     let mut resp_buffer = BytePacketBuffer::new();
     if response.write(&mut resp_buffer).is_err() {
         // Response too large — set TC bit and send header + question only

src/dot.rs

@@ -175,6 +175,7 @@ where
                 resp.header.rescode = ResultCode::SERVFAIL;
                 let mut buf = BytePacketBuffer::new();
                 if resp.write(&mut buf).is_err() {
+                    debug!("DoT: failed to serialize SERVFAIL for {}", remote_addr);
                     break;
                 }
                 buf