Add Order and OrderBook domains with test suites

csharp/tests/OrderTests/OrderCompletionTests.cs

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+using NUnit.Framework;
+using OrderDomain;
+
+namespace OrderTests;
+
+public class OrderCompletionTests
+{
+    [Test]
+    public void CompletedOrder_TotalIsCorrect()
+    {
+        var order = new Order();
+        order.AddLine(100m, 1);
+        order.Complete();
+
+        Assert.That(order.Total, Is.EqualTo(100m));
+    }
+}

csharp/tests/OrderTests/OrderTests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
+    <PackageReference Include="NUnit" Version="4.6.1" />
+    <PackageReference Include="NUnit3TestAdapter" Version="6.3.0" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\OrderDomain\OrderDomain.csproj" />
+  </ItemGroup>
+</Project>

csharp/tests/OrderTests/OrderTotalTests.cs

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+using NUnit.Framework;
+using OrderDomain;
+
+namespace OrderTests;
+
+public class OrderTotalTests
+{
+    [Test]
+    public void DiscountOnFirstLine_TotalAndLineValues()
+    {
+        var order = new Order();
+        order.AddLine(100m, 2);  // line 0: 200 → 160 with discount
+        order.AddLine(50m, 1);   // line 1: 50
+
+        order.ApplyDiscount(0);
+
+        Assert.That(order.Total, Is.EqualTo(210m));
+        Assert.That(order.Lines[0].Value, Is.EqualTo(160m));
+        Assert.That(order.Lines[1].Value, Is.EqualTo(50m));
+    }
+
+    [Test]
+    public void DiscountOnSecondLine_TotalAndLineValues()
+    {
+        var order = new Order();
+        order.AddLine(100m, 2);  // line 0: 200
+        order.AddLine(50m, 1);   // line 1: 50 → 40 with discount
+
+        order.ApplyDiscount(1);
+
+        Assert.That(order.Total, Is.EqualTo(240m));
+        Assert.That(order.Lines[0].Value, Is.EqualTo(200m));
+        Assert.That(order.Lines[1].Value, Is.EqualTo(40m));
+    }
+
+    [Test]
+    public void DiscountOnBothLines_TotalAndLineValues()
+    {
+        var order = new Order();
+        order.AddLine(100m, 2);  // line 0: 200 → 160
+        order.AddLine(50m, 1);   // line 1: 50 → 40
+
+        order.ApplyDiscount(0);
+        order.ApplyDiscount(1);
+
+        Assert.That(order.Total, Is.EqualTo(200m));
+        Assert.That(order.Lines[0].Value, Is.EqualTo(160m));
+        Assert.That(order.Lines[1].Value, Is.EqualTo(40m));
+    }
+
+    [Test]
+    public void NoDiscount_TotalAndLineValues()
+    {
+        var order = new Order();
+        order.AddLine(100m, 2);  // line 0: 200
+        order.AddLine(50m, 1);   // line 1: 50
+
+        Assert.That(order.Total, Is.EqualTo(250m));
+        Assert.That(order.Lines[0].Value, Is.EqualTo(200m));
+        Assert.That(order.Lines[1].Value, Is.EqualTo(50m));
+    }
+
+    [Test]
+    public void SingleLineWithDiscount_TotalAndValue()
+    {
+        var order = new Order();
+        order.AddLine(75m, 4);  // line 0: 300 → 240
+
+        order.ApplyDiscount(0);
+
+        Assert.That(order.Total, Is.EqualTo(240m));
+        Assert.That(order.Lines[0].Value, Is.EqualTo(240m));
+    }
+
+    [Test]
+    public void ThreeLinesOneDiscount_TotalAndLineValues()
+    {
+        var order = new Order();
+        order.AddLine(20m, 1);   // line 0: 20
+        order.AddLine(30m, 2);   // line 1: 60 → 48
+        order.AddLine(10m, 3);   // line 2: 30
+
+        order.ApplyDiscount(1);
+
+        Assert.That(order.Total, Is.EqualTo(98m));
+        Assert.That(order.Lines[0].Value, Is.EqualTo(20m));
+        Assert.That(order.Lines[1].Value, Is.EqualTo(48m));
+        Assert.That(order.Lines[2].Value, Is.EqualTo(30m));
+    }
+}