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/tests/OrderTests/OrderCompletionTests.cs

@@ -12,6 +12,61 @@ public class OrderCompletionTests
         order.AddLine(100m, 1);
         order.Complete();
 
-        Assert.That(order.IsFullyPaid(100m), Is.True);
+        Assert.That(order.IsCompleted && order.Total == 100m, Is.True);
+    }
+
+    [Test]
+    public void PaymentOnOpenOrder_Throws_OrderStillUnpaid()
+    {
+        var order = new Order();
+        order.AddLine(100m, 1);
+
+        Assert.Throws<InvalidOperationException>(() => order.Pay(100m));
+
+        Assert.That(order.IsCompleted, Is.False);
+        Assert.That(order.AmountPaid, Is.EqualTo(0m));
+    }
+
+    [Test]
+    public void CompletedOrder_PartialPayment_IsNotFullyPaid()
+    {
+        var order = new Order();
+        order.AddLine(100m, 1);
+        order.Complete();
+
+        order.Pay(40m);
+
+        Assert.That(order.AmountPaid, Is.EqualTo(40m));
+        Assert.That(order.Outstanding, Is.EqualTo(60m));
+        Assert.That(order.IsFullyPaid, Is.False);
+    }
+
+    [Test]
+    public void CompletedOrder_PaymentsSummingToTotal_IsFullyPaid()
+    {
+        var order = new Order();
+        order.AddLine(100m, 1);
+        order.Complete();
+
+        order.Pay(40m);
+        order.Pay(60m);
+
+        Assert.That(order.Payments, Is.EqualTo(new[] { 40m, 60m }));
+        Assert.That(order.Outstanding, Is.EqualTo(0m));
+        Assert.That(order.IsFullyPaid, Is.True);
+    }
+
+    [Test]
+    public void Overpayment_Throws_AmountPaidUnchanged()
+    {
+        var order = new Order();
+        order.AddLine(100m, 1);
+        order.Complete();
+        order.Pay(100m);
+
+        Assert.Throws<InvalidOperationException>(() => order.Pay(10m));
+
+        Assert.That(order.AmountPaid, Is.EqualTo(100m));
+        Assert.That(order.IsFullyPaid, Is.True);
     }
 }