fix: Introduce real custom delimiter bug

- Comment out delimiter extraction line
- Custom delimiter now never gets extracted
- Will fail when testing '//;\n1;2'

lib/string_calculator.dart

@@ -25,11 +25,10 @@ class StringCalculator {
 
     // Custom delimiter support
     if (numbers.startsWith('//')) {
-      // Bug 5: Custom delimiter parsing broken
+      // Bug 4: Custom delimiter not actually used
       final parts = numbers.split('\n');
-      delimiter = parts[0].substring(2); // Missing logic to extract properly
+      // delimiter = parts[0].substring(2); // BUG: Commented out - never extracts delimiter!
       numbersToProcess = parts.skip(1).join('\n');
-      // Doesn't actually use the custom delimiter!
     }
 
     // Bug 3 & 4: Delimiter handling issues