traverse: apply code review fixes in ResolveNS dispatch loop

- Add RespNSResolutionFailed to the error-type guard in ResolveNS so
  nameserver resolution failures are treated consistently with SERVFAIL
  and RespError (skip + continue rather than fall through).
- Deduplicate nsName extraction before the err/len branches so it is
  computed once instead of twice.

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

internal/traverse/traverser.go

@@ -232,11 +232,11 @@ func (t *Traverser) processReferral(ctx context.Context, ref *Referral, cache *I
 			ref.State = StateResolved
 		} else {
 			addrs, err := t.ResolveNS(ctx, ref.Name, cache, visitedCopy, t.depth)
+			nsName := strings.TrimSuffix(ref.Bailiwick, ".")
+			if nsName == "" || nsName == "." {
+				nsName = strings.TrimSuffix(ref.Name, ".")
+			}
 			if err != nil {
-				nsName := strings.TrimSuffix(ref.Bailiwick, ".")
-				if nsName == "" || nsName == "." {
-					nsName = strings.TrimSuffix(ref.Name, ".")
-				}
 				return &Response{
 					Referral:     ref,
 					Type:         RespNSResolutionFailed,
@@ -247,10 +247,6 @@ func (t *Traverser) processReferral(ctx context.Context, ref *Referral, cache *I
 				ref.Addresses = addrs
 				ref.State = StateResolved
 			} else {
-				nsName := strings.TrimSuffix(ref.Bailiwick, ".")
-				if nsName == "" || nsName == "." {
-					nsName = strings.TrimSuffix(ref.Name, ".")
-				}
 				return &Response{
 					Referral:     ref,
 					Type:         RespNSResolutionFailed,
@@ -371,7 +367,7 @@ func (t *Traverser) ResolveNS(ctx context.Context, nsName string, cache *InfoCac
 			break
 		}
 
-		if resp.Type == RespSERVFAIL || resp.Type == RespError {
+		if resp.Type == RespSERVFAIL || resp.Type == RespError || resp.Type == RespNSResolutionFailed {
 			lastErr = fmt.Errorf("server error resolving %s: %s", nsName, resp.Type)
 			continue
 		}