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/output/stats.go

@@ -138,6 +138,12 @@ func summaryTypeLabel(respType string) string {
 		return "name does not exist"
 	case "servfail":
 		return "resulted in SERVFAIL"
+	case "refused":
+		return "query refused by server"
+	case "notimp":
+		return "query type not implemented by server"
+	case "cname_loop":
+		return "resulted in a CNAME loop"
 	case "error":
 		return "resulted in an error"
 	case "referral":