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/OrderDto.cs

@@ -0,0 +1,21 @@
+namespace After;
+
+/// <summary>
+/// DTO carrying an <see cref="Order"/> — including its back-reference to the
+/// owning <see cref="ClientDto"/> — to the WebApp. A record: value equality
+/// keeps round-trip comparisons trivial.
+/// </summary>
+public sealed record OrderDto
+{
+    /// <summary>Primary key of the underlying order.</summary>
+    public Guid Id { get; init; }
+
+    /// <summary>Free-text description of what this order is for.</summary>
+    public string Description { get; init; } = string.Empty;
+
+    /// <summary>
+    /// Back-reference to the owning client, or <c>null</c> for a bare order
+    /// that has not been added to any client.
+    /// </summary>
+    public ClientDto? ClientDto { get; init; }
+}