Add static Context (Singleton) to DbBase

ActiveRecord pattern: expose a shared DbContext on DbBase so any
entity subclass can reach it without passing context through parameters.
The instance-level DbContext back-reference still works; the static one
provides a class-level global fallback.

tests/BeforeAfter.Tests/BeforeTests.cs

@@ -131,8 +131,7 @@ public class BeforeTests
         Assert.True(app.IsPersisted(client));
     }
 
-    // A companion check: a client that was never attached/saved exposes no
-    // context yet, so the leak is dormant until the entity meets a context.
+    // A companion check: a bare client has no context until saved.
     [Fact]
     public void A_bare_client_has_no_context_until_saved()
     {
@@ -142,4 +141,22 @@ public class BeforeTests
         Assert.Null(client.DbContext);
         Assert.False(app.IsPersisted(client));
     }
+
+    // -----------------------------------------------------------------
+    // (8) Static Context (Singleton): DbBase exposes a class-level
+    //     shared DbContext.  Setting it makes the context accessible
+    //     from any entity via its base type.
+    // -----------------------------------------------------------------
+    [Fact]
+    public void DbBase_has_a_static_Context_singleton()
+    {
+        Assert.Null(DbBase.Context);   // fresh — not set yet
+
+        var db = new DbContext();
+        DbBase.Context = db;
+
+        Assert.Same(db, DbBase.Context);
+
+        DbBase.Context = null;       // cleanup
+    }
 }