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/AfterPersistenceTests.cs

@@ -55,7 +55,7 @@ public class AfterPersistenceTests
         db.Save(order);
 
         order.Description = "one (edited)";
-        order.TotalInCents = 99_00;
+        order.Lines[0].AmountInCents = 99_00;
         client.Name = "Acme GmbH";
         db.Save(order);
         db.Save(client);