feat: implement output formatting and display (HAN-383)

Add text and JSON formatters with real-time progress via traverser hooks,
summary statistics, and CLI integration for --show-* and --json flags.

Co-authored-by: multica-agent <github@multica.ai>

internal/output/text_test.go

@@ -0,0 +1,65 @@
+package output
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/hits/ExploreDNS/internal/dns"
+	"github.com/hits/ExploreDNS/internal/traverse"
+)
+
+func TestTextFormatterProgressIndentation(t *testing.T) {
+	root := traverse.NewReferral("example.com.", dns.TypeA, ".", 0, 1.0, nil)
+	child := traverse.NewReferral("example.com.", dns.TypeA, "com.", 1, 0.5, root)
+
+	var buf bytes.Buffer
+	cfg := DefaultConfig()
+	cfg.Color = false
+	formatter := newTextFormatter(cfg, &buf)
+
+	if err := formatter.WriteProgress(traverse.TraversalEvent{
+		Stage:  traverse.EventStart,
+		Result: traverse.TraversalResult{Referral: child},
+	}); err != nil {
+		t.Fatalf("WriteProgress: %v", err)
+	}
+
+	out := buf.String()
+	if !strings.HasPrefix(out, "  2 ") {
+		t.Fatalf("expected depth-based indentation, got %q", out)
+	}
+}
+
+func TestAttachHooksRespectsShowFlags(t *testing.T) {
+	ref := traverse.NewReferral("example.com.", dns.TypeA, ".", 0, 1.0, nil)
+	var progressCount int
+
+	cfg := DefaultConfig()
+	cfg.ShowProgress = false
+	cfg.ShowResolves = false
+	cfg.ShowAllStats = false
+
+	var buf bytes.Buffer
+	formatter := NewFormatter(cfg, &buf)
+	hooks := AttachHooks(cfg, formatter)
+	hooks.OnEvent(traverse.TraversalEvent{
+		Stage:  traverse.EventStart,
+		Result: traverse.TraversalResult{Referral: ref},
+	})
+
+	if buf.Len() != 0 {
+		t.Fatalf("expected no output when ShowProgress is false")
+	}
+
+	cfg.ShowProgress = true
+	hooks = AttachHooks(cfg, formatter)
+	hooks.OnEvent(traverse.TraversalEvent{
+		Stage:  traverse.EventStart,
+		Result: traverse.TraversalResult{Referral: ref},
+	})
+	progressCount = strings.Count(buf.String(), "\n")
+	if progressCount == 0 {
+		t.Fatal("expected progress output when ShowProgress is true")
+	}
+}