Add round 2: split multi-step OrderBook tests (example created by participant)

- Add OrderBookDomain: in-memory order book with Pending →
  WaitingForPayment → Fulfilling → Delivered state machine, payment
  window based on passed-in timestamps, cancellation rules
- Add OrderBookTests: 7 integration tests in given/when/then/when/then
  anti-pattern style as starting material for the exercise
- Learning hour: add round 2 where participants create the example
  themselves before showing it to the agent (no refactored test given)
- Fix sln: OrderTests Debug config was mapped to Release

exercises/show-dont-tell/src/OrderBookDomain/Order.cs

@@ -0,0 +1,25 @@
+namespace OrderBookDomain;
+
+public enum OrderStatus
+{
+    Pending,
+    WaitingForPayment,
+    Fulfilling,
+    Delivered,
+    Cancelled
+}
+
+public class Order
+{
+    public int Id { get; }
+    public string Customer { get; }
+    public OrderStatus Status { get; internal set; }
+    public DateTimeOffset? FinalizedAt { get; internal set; }
+
+    internal Order(int id, string customer)
+    {
+        Id = id;
+        Customer = customer;
+        Status = OrderStatus.Pending;
+    }
+}