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/Order.cs

@@ -0,0 +1,26 @@
+namespace After;
+
+/// <summary>
+/// An order as a plain domain object: no <see cref="DbBase"/> inheritance, no
+/// knowledge of <see cref="DbContext"/>. Its data travels to the WebApp as an
+/// <see cref="OrderDto"/>.
+/// </summary>
+public class Order
+{
+    /// <summary>
+    /// Primary key as a plain value: <see cref="Guid.Empty"/> until a
+    /// <see cref="DbContext"/> explicitly saves this order (which copies a
+    /// generated id in).
+    /// </summary>
+    public Guid Id { get; set; }
+
+    /// <summary>Free-text description of what this order is for.</summary>
+    public string Description { get; set; } = string.Empty;
+
+    /// <summary>
+    /// The client that owns this order. Set automatically by
+    /// <see cref="ClientOrders"/> when the order is added to
+    /// <see cref="Client.Orders"/> (navigation fix-up).
+    /// </summary>
+    public Client? Client { get; set; }
+}