Yak: csharp before situation (Yak 01) — DB-backed domain classes

Add the C# BEFORE situation: a fake-EF DbContext + DbBase that Client/Order
inherit, the active-record DbContext back-reference (the dependency leak),
navigation fix-up on Client.Orders, and a WebApp that consumes the domain
classes directly. Adds an empty After project so the solution shape is final,
and an xUnit BeforeTests suite covering the required assertions.

- db-subclass-to-dto.sln (classic .sln; SDK 10 defaults to .slnx which breaks run-tests.sh)
- src/Before: DbBase, DbContext, Client (+ClientOrders fix-up), Order, WebApp
- src/After: empty placeholder (filled in Yak 02)
- tests/BeforeAfter.Tests/BeforeTests.cs: 8 tests (7 required + 1 companion)

src/Before/Client.cs

@@ -0,0 +1,50 @@
+using System.Collections.ObjectModel;
+
+namespace Before;
+
+/// <summary>
+/// A client, backed by the database: it derives from <see cref="DbBase"/> and
+/// holds its orders as a navigation collection. In the "after" situation this
+/// becomes a plain POCO (no <see cref="DbBase"/>) with a <c>ClientDto</c>
+/// carrying its data to the WebApp.
+/// </summary>
+public class Client : DbBase
+{
+    /// <summary>Display name of the client.</summary>
+    public string Name { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Navigation collection of this client's orders. Adding to it performs
+    /// 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);
+    }
+}