Exercise 2 solution: OrderLines in the domain, edges unchanged

OrderLine + computed TotalInCents; both inbound mappers reconstruct a
one-line order from the flat DTO/row (derived data is re-derived, not
written); builder realises WithTotal as a single line. Outbound mappers,
DTO, OrderData, DbContext and WebApp untouched — the small-steps payoff.

yak: exercise-2-solution-on-branch-exercise-2-solution-hw7e

tests/BeforeAfter.Tests/OrderLinesEvolutionTests.cs

@@ -0,0 +1,51 @@
+using After;
+using static BeforeAfter.Tests.TestData.ClientBuilder;
+using static BeforeAfter.Tests.TestData.OrderBuilder;
+
+namespace BeforeAfter.Tests;
+
+/// <summary>
+/// EXERCISE 2 — the domain gained OrderLines; persistence still stores one
+/// flat row per order. (This is the solution participants arrive at from the
+/// stub in EXERCISE2.md.)
+/// </summary>
+public class OrderLinesEvolutionTests
+{
+    [Fact] // EXERCISE STEP 3a
+    public void A_reloaded_order_can_be_discounted()
+    {
+        var db = new DbContext();
+        var order = ValidOrder().WithDescription("big order").WithTotal(150_00).Build();
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
+        db.Save(client);
+        db.Save(order);
+
+        var loaded = db.LoadOrder(order.Id)!;
+
+        // The discount works off the computed total of the RELOADED order:
+        var discounted = DiscountCalculator.CalculateDiscount(
+            loaded, discountBarrierCents: 100_00, discountPercentage: 10);
+
+        Assert.Equal(135_00, discounted.DiscountedTotalInCents);
+        Assert.Equal(10, discounted.DiscountPercentage);
+    }
+
+    [Fact] // EXERCISE STEP 3b — the accepted loss
+    public void A_multi_line_order_flattens_to_a_single_line_when_stored()
+    {
+        var db = new DbContext();
+        var order = ValidOrder().WithDescription("mixed order").Build();
+        order.Lines.Add(new OrderLine("first item", 100_00));
+        order.Lines.Add(new OrderLine("second item", 50_00));
+
+        db.Save(order);
+
+        var loaded = db.LoadOrder(order.Id)!;
+
+        // Storage flattened the order: one line, the flat total, and the
+        // order-level description. Documented, not hidden.
+        var line = Assert.Single(loaded.Lines);
+        Assert.Equal(150_00, line.AmountInCents);
+        Assert.Equal("mixed order", line.Description);
+    }
+}