feat(duckduckgo): detect anti-bot challenge and surface ErrBlocked

DuckDuckGo intermittently serves a CAPTCHA modal ("Unfortunately, bots
use DuckDuckGo too...") instead of search results. The result selector
matches zero elements on that page, so callers used to get
([]Result{}, nil) — silent empty results that look like "no matches."

Detect the challenge via the BEM class .anomaly-modal__title and return
a typed ErrBlocked so callers can distinguish blocked from no-results
and react (retry, fallback to another engine, surface to user).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

sites/duckduckgo/extract_test.go

@@ -2,6 +2,7 @@ package duckduckgo
 
 import (
 	"context"
+	"errors"
 	"testing"
 
 	"gitea.stevedudenhoeffer.com/steve/go-extractor"
@@ -106,6 +107,26 @@ func TestExtractResults_NoLinks(t *testing.T) {
 	}
 }
 
+func TestExtractResults_Blocked(t *testing.T) {
+	doc := &extractortest.MockDocument{
+		MockNode: extractortest.MockNode{
+			Children: map[string]extractor.Nodes{
+				".anomaly-modal__title": {
+					&extractortest.MockNode{TextValue: "Unfortunately, bots use DuckDuckGo too."},
+				},
+			},
+		},
+	}
+
+	results, err := extractResults(doc)
+	if !errors.Is(err, ErrBlocked) {
+		t.Fatalf("expected ErrBlocked, got %v", err)
+	}
+	if results != nil {
+		t.Errorf("expected nil results when blocked, got %v", results)
+	}
+}
+
 func TestSearch_UsesMockBrowser(t *testing.T) {
 	doc := &extractortest.MockDocument{
 		URLValue: "https://duckduckgo.com/?q=test",