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

lib/string_calculator.dart

@@ -0,0 +1,51 @@
+/// String Calculator - Buggy Implementation
+/// This code has intentional bugs for the Bug Hunt Kata exercise
+///
+/// Requirements:
+/// 1. Empty string returns 0
+/// 2. Single number returns that number
+/// 3. Two numbers comma-delimited returns sum
+/// 4. Handle newlines as delimiters
+/// 5. Support custom delimiters: "//[delimiter]\n[numbers]"
+
+class StringCalculator {
+  int add(String numbers) {
+    // Bug 1: Empty string handling
+    if (numbers.isEmpty) {
+      return 1; // Should return 0
+    }
+
+    // Bug 2: Single number parsing
+    if (!numbers.contains(',') && !numbers.contains('\n') && !numbers.startsWith('//')) {
+      return int.parse(numbers) + 1; // Off by one error
+    }
+
+    String delimiter = ',';
+    String numbersToProcess = numbers;
+
+    // Custom delimiter support
+    if (numbers.startsWith('//')) {
+      // Bug 5: Custom delimiter parsing broken
+      final parts = numbers.split('\n');
+      delimiter = parts[0].substring(2); // Missing logic to extract properly
+      numbersToProcess = parts.skip(1).join('\n');
+      // Doesn't actually use the custom delimiter!
+    }
+
+    // Bug 3 & 4: Delimiter handling issues
+    final numList = numbersToProcess
+        .replaceAll('\n', delimiter)
+        .split(delimiter)
+        .where((s) => s.isNotEmpty)
+        .map((s) => int.parse(s))
+        .toList();
+
+    // Bug 3: Off-by-one in summation
+    int sum = 0;
+    for (int i = 0; i < numList.length - 1; i++) { // Misses last element!
+      sum += numList[i];
+    }
+
+    return sum;
+  }
+}