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/TestData/OrderBuilder.cs

@@ -16,6 +16,7 @@ namespace BeforeAfter.Tests.TestData;
 public sealed class OrderBuilder
 {
     private readonly Order _order = new();
+    private int? _totalInCents;
 
     /// <summary>A valid, fresh order — the neutral starting point for tests.</summary>
     public static OrderBuilder ValidOrder() => new();
@@ -33,15 +34,23 @@ public sealed class OrderBuilder
     }
 
     /// <summary>
-    /// Sets the total in cents (<c>150_00</c> = €150.00). How the requested
-    /// total is realised inside the order is an implementation detail that may
-    /// change as the domain evolves; this API stays.
+    /// Sets the total in cents (<c>150_00</c> = €150.00). Since OrderLines
+    /// arrived, the total is computed — the requested total is realised in
+    /// <see cref="Build"/> as a single line carrying the order's description,
+    /// so <c>WithDescription</c>/<c>WithTotal</c> work in any order. This API
+    /// stayed identical across that domain evolution; only these internals
+    /// changed.
     /// </summary>
     public OrderBuilder WithTotal(int totalInCents)
     {
-        _order.TotalInCents = totalInCents;
+        _totalInCents = totalInCents;
         return this;
     }
 
-    public Order Build() => _order;
+    public Order Build()
+    {
+        if (_totalInCents is int total)
+            _order.Lines.Add(new OrderLine(_order.Description, total));
+        return _order;
+    }
 }