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/DbBase.cs

@@ -0,0 +1,29 @@
+namespace Before;
+
+/// <summary>
+/// Base class for every DB-backed domain object. Fakes the active-record part
+/// of EF: each entity carries its own <see cref="Id"/> and, once it has been
+/// created/saved through a context, a back-reference to that context.
+///
+/// <see cref="DbContext"/> is the dependency this base class leaks. The
+/// "after" situation (Yak 02) removes this inheritance entirely — domain
+/// objects become plain POCOs that know nothing about a DbContext.
+/// </summary>
+public class DbBase
+{
+    /// <summary>
+    /// Primary key. Fresh (unsaved) entities have <see cref="Guid.Empty"/>;
+    /// <see cref="DbContext.Save"/> assigns a real id, faking EF's identity
+    /// generation.
+    /// </summary>
+    public Guid Id { get; set; }
+
+    /// <summary>
+    /// Active-record back-reference: "I know which context created me". This
+    /// is the leak the exercise exposes — from a plain domain object you can
+    /// reach straight into the persistence layer. <c>internal set</c> because
+    /// only the owning <see cref="DbContext"/> may (re)assign it; consumers
+    /// (including the WebApp, and the tests) can only read it.
+    /// </summary>
+    public DbContext? DbContext { get; internal set; }
+}