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/traverser.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"net"
 	"sync"
+	"time"
 
 	"github.com/hits/ExploreDNS/internal/dns"
 	miekgdns "github.com/miekg/dns"
@@ -141,7 +142,19 @@ func (t *Traverser) Traverse(ctx context.Context, name string) ([]TraversalResul
 		if resp.Type == RespCNAMEFollow {
 			follow := resp.CNAMEFollowReferral()
 			if follow != nil {
-				if !stack.Push(follow) {
+				// Detect CNAME loop: target name already appears in the ancestor chain.
+				if follow.Parent != nil && follow.Parent.IsNameInChain(follow.Name) {
+					mu.Lock()
+					results = append(results, TraversalResult{
+						Referral: follow,
+						Response: &Response{
+							Referral:     follow,
+							Type:         RespCNAMELoop,
+							ErrorMessage: fmt.Sprintf("CNAME loop detected: %s already in traversal chain", follow.Name),
+						},
+					})
+					mu.Unlock()
+				} else if !stack.Push(follow) {
 					mu.Lock()
 					results = append(results, TraversalResult{
 						Referral: follow,
@@ -412,12 +425,16 @@ func (t *Traverser) resolveGlueViaSystem(ctx context.Context, name string, cache
 
 	c := &miekgdns.Client{
 		Net:          "udp",
-		ReadTimeout:  5,
-		WriteTimeout: 5,
+		ReadTimeout:  5 * time.Second,
+		WriteTimeout: 5 * time.Second,
 	}
 	if deadline, ok := ctx.Deadline(); ok {
-		c.ReadTimeout = deadline.Sub(deadline)
-		c.WriteTimeout = deadline.Sub(deadline)
+		remaining := time.Until(deadline)
+		if remaining <= 0 {
+			return nil
+		}
+		c.ReadTimeout = remaining
+		c.WriteTimeout = remaining
 	}
 
 	fqdn := miekgdns.Fqdn(name)