GREEN: Fix custom delimiter bug

- Uncommented delimiter extraction line
- Now properly extracts custom delimiter from format '//[delimiter]\n'
- Tests pass: '//;\n1;2' returns 3, '//|\n10|20|30' returns 60

lib/string_calculator.dart

@@ -27,7 +27,7 @@ class StringCalculator {
     if (numbers.startsWith('//')) {
       // Bug 4: Custom delimiter not actually used
       final parts = numbers.split('\n');
-      // delimiter = parts[0].substring(2); // BUG: Commented out - never extracts delimiter!
+      delimiter = parts[0].substring(2); // Fixed: Extract custom delimiter
       numbersToProcess = parts.skip(1).join('\n');
     }