fix: address code review issues on PR #9

- gofmt main.go to fix tab indentation in main() body
- stats.go ComputeSummary: return nil when Answers and ByType are empty
- dns/types.go: remove duplicate ParseQueryType (dead code)
- formatter.go: log hook errors to stderr when Debug > 0; add Debug to Config
- text.go referralID: remove redundant depth==0 branch
- text.go writeServers: remove version lookup placeholder string

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

internal/output/formatter.go

@@ -1,6 +1,7 @@
 package output
 
 import (
+	"fmt"
 	"io"
 	"os"
 
@@ -28,6 +29,7 @@ type Config struct {
 	Verbose            bool
 	Quiet              bool
 	Color              bool
+	Debug              int
 }
 
 func DefaultConfig() *Config {
@@ -69,16 +71,21 @@ func AttachHooks(cfg *Config, formatter Formatter) *traverse.TraverserHooks {
 	if cfg == nil || formatter == nil {
 		return nil
 	}
+	logErr := func(context string, err error) {
+		if err != nil && cfg.Debug > 0 {
+			fmt.Fprintf(os.Stderr, "Debug: formatter %s: %v\n", context, err)
+		}
+	}
 	return &traverse.TraverserHooks{
 		OnEvent: func(event traverse.TraversalEvent) {
 			switch {
 			case event.IsResolve && cfg.ShowResolves:
-				_ = formatter.WriteResolve(event)
+				logErr("WriteResolve", formatter.WriteResolve(event))
 			case !event.IsResolve && cfg.ShowProgress:
-				_ = formatter.WriteProgress(event)
+				logErr("WriteProgress", formatter.WriteProgress(event))
 			}
 			if event.Stage == traverse.EventComplete && cfg.ShowAllStats {
-				_ = formatter.WriteResult(event.Result)
+				logErr("WriteResult", formatter.WriteResult(event.Result))
 			}
 		},
 	}