Add Order and OrderBook domains with test suites

csharp/src/OrderDomain/Order.cs

@@ -0,0 +1,18 @@
+namespace OrderDomain;
+
+public class Order
+{
+    public List<OrderLine> Lines { get; } = new();
+
+    public decimal Total => Lines.Sum(l => l.Value);
+
+    public bool IsCompleted { get; private set; }
+
+    public void AddLine(decimal unitPrice, int quantity)
+        => Lines.Add(new OrderLine(unitPrice, quantity));
+
+    public void ApplyDiscount(int lineIndex)
+        => Lines[lineIndex].ApplyDiscount();
+
+    public void Complete() => IsCompleted = true;
+}