feat: Add buggy String Calculator implementation

- Implements basic string calculator with 5 intentional bugs
- Bug 1: Empty string returns 1 instead of 0
- Bug 2: Single number has off-by-one error
- Bug 3: Summation loop misses last element
- Bug 4: Newline delimiter not properly handled
- Bug 5: Custom delimiter parsing broken
- Ready for Bug Hunt Kata with TDD approach

test/string_calculator_test.dart

@@ -0,0 +1,14 @@
+import 'package:test/test.dart';
+import 'package:tdd_katas/string_calculator.dart';
+
+void main() {
+  group('String Calculator - Bug Hunt', () {
+    late StringCalculator calculator;
+
+    setUp(() {
+      calculator = StringCalculator();
+    });
+
+    // Tests will be added as we hunt bugs
+  });
+}