Initial commit: Complete TDD workshop materials

- Workshop documentation (WORKSHOP_PLAN, FACILITATOR_GUIDE, etc.)
- FizzBuzz kata with demo script (git history to be recreated)
- Password Validator kata with demo script and solution
- Shopping Cart kata with demo script and solution
- Setup guide and TDD reference card for participants

shopping_cart/lib/shopping_cart.dart

@@ -0,0 +1,46 @@
+// TODO: Implement ShoppingCart using TDD.
+//
+// Domain rules (implement one at a time, test first!):
+//   1. Add an item (name, price, quantity) to the cart
+//   2. Get the subtotal (sum of price × quantity for all items)
+//   3. Remove an item by name
+//   4. Get total item count (sum of all quantities)
+//   5. Apply a percentage discount to the subtotal (e.g. 10% off)
+//   6. Items with zero or negative price are rejected
+//
+// CartItem is a Value Object — immutable, validated at construction.
+// ShoppingCart is stateful — it accumulates items across calls.
+
+class CartItem {
+  final String name;
+  final double price;
+  final int quantity;
+
+  CartItem({required this.name, required this.price, required this.quantity}) {
+    // TODO: validate price > 0 and quantity > 0
+  }
+}
+
+class ShoppingCart {
+  // TODO: implement
+
+  void addItem(CartItem item) {
+    throw UnimplementedError();
+  }
+
+  void removeItem(String name) {
+    throw UnimplementedError();
+  }
+
+  double subtotal() {
+    throw UnimplementedError();
+  }
+
+  int itemCount() {
+    throw UnimplementedError();
+  }
+
+  double totalAfterDiscount(double discountPercent) {
+    throw UnimplementedError();
+  }
+}