Initial commit: TDD Workshop exercises for participants

- Password Validator kata with starter code and tests
- Shopping Cart kata with starter code and tests
- FizzBuzz reference code (from live demo)
- Setup guide and TDD reference card
- No solutions included (participants implement themselves)

password_validator/lib/password_validator.dart

@@ -0,0 +1,25 @@
+// TODO: Implement PasswordValidator using TDD.
+//
+// Rules (implement one at a time, test first!):
+//   1. Must be at least 8 characters long
+//   2. Must contain at least one uppercase letter
+//   3. Must contain at least one digit
+//   4. Must contain at least one special character (!@#$%^&*)
+//   5. Must not contain spaces
+//
+// ValidationResult holds a pass/fail flag AND a list of error messages.
+// Never return a boolean — the caller deserves to know *why* it failed.
+
+class ValidationResult {
+  final bool isValid;
+  final List<String> errors;
+
+  const ValidationResult({required this.isValid, required this.errors});
+}
+
+class PasswordValidator {
+  ValidationResult validate(String password) {
+    // TODO: implement
+    throw UnimplementedError();
+  }
+}