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/dns/query.go

@@ -93,7 +93,7 @@ func QueryWithExchange(ctx context.Context, server net.IP, name string, qtype ui
 			select {
 			case <-ctx.Done():
 				return nil, fmt.Errorf("query retries cancelled: %w", ctx.Err())
-			case <-time.After(100 * time.Millisecond):
+			case <-time.After(backoffDelay(attempt)):
 			}
 		}
 
@@ -156,7 +156,7 @@ func IterativeQueryWithExchange(ctx context.Context, server net.IP, name string,
 			select {
 			case <-ctx.Done():
 				return nil, fmt.Errorf("query retries cancelled: %w", ctx.Err())
-			case <-time.After(100 * time.Millisecond):
+			case <-time.After(backoffDelay(attempt)):
 			}
 		}
 
@@ -195,6 +195,20 @@ func IterativeQueryWithExchange(ctx context.Context, server net.IP, name string,
 	return nil, fmt.Errorf("iterative query %s %s failed after %d retries: %w", name, QNameType(qtype), cfg.Retries, lastErr)
 }
 
+// backoffDelay computes the wait duration before the given retry attempt (1-indexed).
+// Delays: attempt=1 → 100ms, attempt=2 → 200ms, attempt=3 → 400ms, capped at 2s.
+func backoffDelay(attempt int) time.Duration {
+	if attempt <= 0 {
+		return 0
+	}
+	delay := time.Duration(uint(1)<<uint(attempt-1)) * 100 * time.Millisecond
+	const maxDelay = 2 * time.Second
+	if delay > maxDelay {
+		return maxDelay
+	}
+	return delay
+}
+
 func buildQuery(name string, qtype uint16, udpSize int) *dns.Msg {
 	m := new(dns.Msg)
 	m.SetQuestion(dns.Fqdn(name), qtype)