Add payment state to Order with guarded Pay transition

- Order records payments: Payments, AmountPaid, Outstanding,
  IsFullyPaid (parameterless, replaces IsFullyPaid(expectedTotal))
- Pay rejects open orders, non-positive amounts, and overpayments
- 4 new OrderCompletion tests assert observable state:
  PaymentOnOpenOrder_Throws_OrderStillUnpaid,
  CompletedOrder_PartialPayment_IsNotFullyPaid,
  CompletedOrder_PaymentsSummingToTotal_IsFullyPaid,
  Overpayment_Throws_AmountPaidUnchanged
- 11 tests green (6 OrderTotal + 5 OrderCompletion)

csharp/src/OrderDomain/Order.cs

@@ -2,12 +2,22 @@ namespace OrderDomain;
 
 public class Order
 {
+    private readonly List<decimal> payments = new();
+
     public List<OrderLine> Lines { get; } = new();
 
     public decimal Total => Lines.Sum(l => l.Value);
 
     public bool IsCompleted { get; private set; }
 
+    public IReadOnlyList<decimal> Payments => payments;
+
+    public decimal AmountPaid => payments.Sum();
+
+    public decimal Outstanding => Total - AmountPaid;
+
+    public bool IsFullyPaid => IsCompleted && Outstanding == 0m;
+
     public void AddLine(decimal unitPrice, int quantity)
         => Lines.Add(new OrderLine(unitPrice, quantity));
 
@@ -16,6 +26,14 @@ public class Order
 
     public void Complete() => IsCompleted = true;
 
-    public bool IsFullyPaid(decimal expectedTotal)
-        => IsCompleted && Total == expectedTotal;
+    public void Pay(decimal amount)
+    {
+        if (!IsCompleted)
+            throw new InvalidOperationException("Cannot pay an open order.");
+        if (amount <= 0m)
+            throw new ArgumentOutOfRangeException(nameof(amount), amount, "Payment must be positive.");
+        if (amount > Outstanding)
+            throw new InvalidOperationException("Payment exceeds outstanding amount.");
+        payments.Add(amount);
+    }
 }