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/page.go

@@ -18,9 +18,14 @@ type searchPage struct {
 }
 
 func (s searchPage) GetResults() ([]Result, error) {
+	return extractResults(s.doc)
+}
+
+// extractResults parses search results from a DuckDuckGo results page.
+func extractResults(doc extractor.Node) ([]Result, error) {
 	var res []Result
 
-	err := s.doc.ForEach(`article[id^="r1-"]`, func(n extractor.Node) error {
+	err := doc.ForEach(`article[id^="r1-"]`, func(n extractor.Node) error {
 		var r Result
 
 		links := n.Select(`a[href][target="_self"]`)