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/README.md

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+# Feature B — Shopping Cart
+
+## Your goal
+
+Implement `ShoppingCart` using strict TDD. **Do not write any production code before you have a failing test.**
+
+## Domain rules to implement (in order)
+
+| Step | Feature | Notes |
+|------|---------|-------|
+| 1 | Empty cart has subtotal 0 and count 0 | The baseline |
+| 2 | Add items — subtotal = price × quantity | Multiple items accumulate |
+| 3 | Remove item by name | Removing unknown name does nothing |
+| 4 | Apply % discount to subtotal | 10% off $50 → $45 |
+| 5 | CartItem rejects zero/negative price & quantity | Value Object validation |
+| 6 | Bonus: duplicate name / float precision | Design decision — your call |
+
+## The rhythm — repeat for every rule
+
+```
+1. Uncomment the next test → run → watch it FAIL (Red)
+2. Write the minimum code to make it pass → run → GREEN
+3. Refactor if needed → run → still GREEN
+4. Move to the next test
+```
+
+## Running the tests
+
+```bash
+dart pub get
+dart test
+```
+
+## Files
+
+- `lib/shopping_cart.dart` — your implementation goes here
+- `test/shopping_cart_test.dart` — uncomment tests one at a time
+
+## Design hints (don't peek until you're stuck!)
+
+<details>
+<summary>Hint for Step 3 — Remove by name</summary>
+
+When you need to remove by name, a `List<CartItem>` forces you to search by name.
+A `Map<String, CartItem>` makes lookup O(1) and removal trivial.
+Let the test drive you toward the right data structure.
+</details>
+
+<details>
+<summary>Hint for Step 5 — Value Object</summary>
+
+Validation belongs in `CartItem`'s constructor, not in `ShoppingCart.addItem()`.
+If the item can't be created, the cart never sees it.
+This is the Value Object pattern: invalid state is unrepresentable.
+</details>
+
+<details>
+<summary>Hint for the float precision bonus test</summary>
+
+Use `closeTo(expected, delta)` instead of `equals()` for floating point comparisons.
+Example: `expect(result, closeTo(0.30, 0.001))`
+</details>