refactor: deduplicate numericOnly and DuckDuckGo result extraction

- Extract identical numericOnly inline functions from powerball and
  megamillions into shared sites/internal/parse.NumericOnly with tests
- Extract duplicated DuckDuckGo result parsing from Search() and
  GetResults() into shared extractResults() helper

Closes #13, #14

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sites/duckduckgo/duckduckgo.go

@@ -95,39 +95,5 @@ func (c Config) Search(ctx context.Context, b extractor.Browser, query string) (
 	}
 	defer extractor.DeferClose(doc)
 
-	var res []Result
-
-	err = doc.ForEach(`article[id^="r1-"]`, func(n extractor.Node) error {
-		var r Result
-
-		links := n.Select(`a[href][target="_self"]`)
-
-		if len(links) == 0 {
-			return nil
-		}
-
-		r.URL, err = links[0].Attr(`href`)
-
-		if err != nil {
-			return fmt.Errorf("failed to get link: %w", err)
-		}
-
-		titles := n.Select("h2")
-
-		if len(titles) != 0 {
-			r.Title, _ = titles[0].Text()
-		}
-
-		descriptions := n.Select("span > span")
-
-		if len(descriptions) != 0 {
-			r.Description, _ = descriptions[0].Text()
-		}
-
-		res = append(res, r)
-
-		return nil
-	})
-
-	return res, err
+	return extractResults(doc)
 }