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

fizzbuzz/lib/fizzbuzz.dart

@@ -0,0 +1,8 @@
+// FizzBuzz kata - TDD progression
+
+String fizzBuzz(int n) {
+  if (n % 15 == 0) return 'FizzBuzz';
+  if (n % 3 == 0) return 'Fizz';
+  if (n % 5 == 0) return 'Buzz';
+  return n.toString();
+}