Yak: csharp after situation (Yak 02) — plain domain + DTOs + mappers

After/: plain Client/Order (no DbBase), ClientDto/OrderDto records
carrying the relations both ways, hand-written Mappers with a
reference-identity cache for the back-reference cycle, DbContext with
explicit Save(domain) via shadow entities (flipped dependency), and a
WebApp that consumes only DTOs. AfterTests cover the 7 required
behaviours; full solution builds with 0 warnings, 26/26 tests pass.

src/After/Client.cs

@@ -0,0 +1,58 @@
+using System.Collections.ObjectModel;
+
+namespace After;
+
+/// <summary>
+/// A plain domain object — the "after" shape of the exercise: no
+/// <see cref="DbBase"/> inheritance, no knowledge of <see cref="DbContext"/>.
+/// Its data travels to the WebApp as a <see cref="ClientDto"/>; persisting it
+/// is an explicit act of the context, not an inherited capability.
+/// </summary>
+public class Client
+{
+    /// <summary>
+    /// Primary key as a plain value: fresh objects have
+    /// <see cref="Guid.Empty"/> until a <see cref="DbContext"/> explicitly
+    /// saves them (which copies a generated id in). A plain field — it is not
+    /// a persistence concept leaking into the domain.
+    /// </summary>
+    public Guid Id { get; set; }
+
+    /// <summary>Display name of the client.</summary>
+    public string Name { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Navigation collection of this client's orders, with EF-style
+    /// navigation fix-up (see <see cref="ClientOrders"/>).
+    /// </summary>
+    public ClientOrders Orders { get; }
+
+    public Client()
+    {
+        Orders = new ClientOrders(this);
+    }
+}
+
+/// <summary>
+/// An <see cref="Order"/> collection that fakes EF's navigation fix-up: when an
+/// order is added, its <see cref="Order.Client"/> back-reference is set to the
+/// owning client, exactly as EF would wire up the two ends of the relation.
+/// </summary>
+public sealed class ClientOrders : Collection<Order>
+{
+    private readonly Client _owner;
+
+    public ClientOrders(Client owner)
+    {
+        _owner = owner;
+    }
+
+    // Both Add(...) and Insert(...) funnel through InsertItem, so overriding
+    // it covers every way an order can be added to the collection.
+    protected override void InsertItem(int index, Order item)
+    {
+        ArgumentNullException.ThrowIfNull(item);
+        item.Client = _owner;   // navigation fix-up: order now knows its client
+        base.InsertItem(index, item);
+    }
+}