Phase 4.1: Error handling, edge cases, and robustness

- Exponential backoff retry logic (100ms, 200ms, 400ms... capped at 2s)
  replacing fixed 100ms delay between retries
- Explicit REFUSED and NOTIMP response types (RespREFUSED, RespNOTIMPL)
  surfaced as terminal results with user-visible messages
- CNAME loop detection: walking the ancestor referral chain before
  following a CNAME prevents infinite recursion; produces RespCNAMELoop
- DNAME record support: synthesize CNAME target from DNAME mapping when
  the server omits the RFC 6672 synthesized CNAME record
- ErrorMessage field on Response for surfacing error details to users
- Fix resolveGlueViaSystem timeout bug: deadline.Sub(deadline) was
  always 0; replaced with time.Until(deadline)
- DNAME records excluded from hasFinalAnswer so DNAME-only responses
  are correctly classified as RespCNAMEFollow
- Text and JSON output updated with labels for all new response types
- Tests: CNAME loop (2-step and direct), REFUSED, NOTIMP, graceful
  degradation (partial and total server failure), DNAME synthesis,
  IsNameInChain, backoffDelay, ResponseClassification strings

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: multica-agent <github@multica.ai>

internal/traverse/response.go

@@ -16,6 +16,9 @@ const (
 	RespNODATA
 	RespNXDOMAIN
 	RespSERVFAIL
+	RespREFUSED
+	RespNOTIMPL
+	RespCNAMELoop
 	RespError
 )
 
@@ -33,6 +36,12 @@ func (rt ResponseType) String() string {
 		return "nxdomain"
 	case RespSERVFAIL:
 		return "servfail"
+	case RespREFUSED:
+		return "refused"
+	case RespNOTIMPL:
+		return "notimp"
+	case RespCNAMELoop:
+		return "cname_loop"
 	case RespError:
 		return "error"
 	default:
@@ -41,11 +50,12 @@ func (rt ResponseType) String() string {
 }
 
 type Response struct {
-	Referral *Referral
-	Server   net.IP
-	Cache    *InfoCache
-	Decoded  *dns.DecodedResponse
-	Type     ResponseType
+	Referral     *Referral
+	Server       net.IP
+	Cache        *InfoCache
+	Decoded      *dns.DecodedResponse
+	Type         ResponseType
+	ErrorMessage string
 }
 
 func NewResponse(ref *Referral, server net.IP, cache *InfoCache) *Response {
@@ -59,15 +69,28 @@ func NewResponse(ref *Referral, server net.IP, cache *InfoCache) *Response {
 func (r *Response) Process(msg *miekgdns.Msg) *Response {
 	if msg == nil {
 		r.Type = RespError
+		r.ErrorMessage = "nil DNS response"
 		return r
 	}
 
 	r.Decoded = dns.DecodeResponse(msg)
 	if r.Decoded == nil {
 		r.Type = RespError
+		r.ErrorMessage = "failed to decode DNS response"
 		return r
 	}
 
+	// Synthesize CNAME from DNAME when the server didn't include a synthesized CNAME record.
+	if len(r.Decoded.CNAMEChain) == 0 && r.Referral != nil && len(r.Decoded.DNAMEMappings) > 0 {
+		for _, dm := range r.Decoded.DNAMEMappings {
+			synthesized := dns.SynthesizeCNAMEFromDNAME(r.Referral.Name, dm.Owner, dm.Target)
+			if synthesized != "" {
+				r.Decoded.CNAMEChain = append(r.Decoded.CNAMEChain, synthesized)
+				break
+			}
+		}
+	}
+
 	r.Type = r.classify()
 	return r
 }
@@ -78,6 +101,10 @@ func (r *Response) classify() ResponseType {
 		return RespNXDOMAIN
 	case dns.ResponseSERVFAIL:
 		return RespSERVFAIL
+	case dns.ResponseREFUSED:
+		return RespREFUSED
+	case dns.ResponseNOTIMPL:
+		return RespNOTIMPL
 	case dns.ResponseAnswer:
 		if len(r.Decoded.CNAMEChain) > 0 && !r.hasFinalAnswer() {
 			return RespCNAMEFollow
@@ -97,6 +124,9 @@ func (r *Response) hasFinalAnswer() bool {
 		if _, ok := rr.(*miekgdns.CNAME); ok {
 			continue
 		}
+		if _, ok := rr.(*miekgdns.DNAME); ok {
+			continue
+		}
 		return true
 	}
 	return false
@@ -200,7 +230,7 @@ func (r *Response) resolveGlue(child *Referral) {
 
 func (r *Response) IsTerminal() bool {
 	switch r.Type {
-	case RespAnswer, RespNODATA, RespNXDOMAIN, RespSERVFAIL, RespError:
+	case RespAnswer, RespNODATA, RespNXDOMAIN, RespSERVFAIL, RespREFUSED, RespNOTIMPL, RespCNAMELoop, RespError:
 		return true
 	default:
 		return false