Find falls back to DbBase.Context singleton when no context passed

DbBase.Find<T> now uses db ?? Context, and Client.FindByName's context
parameter becomes optional — call sites read like Client.FindByName("Jane
Doe") without context threading. A null result now overwhelmingly means
"no match"; the residual "no context configured at all" ambiguity is
documented in remarks as part of the ActiveRecord-leak cost this exercise
illustrates.

tests/BeforeAfter.Tests/BeforeTests.cs

@@ -229,7 +229,8 @@ public class BeforeTests
     }
 
     // ---------------------------------------------------------------------
-    // (13) DbBase.Find handles a null context gracefully.
+    // (13) DbBase.Find handles a null context gracefully — with no explicit
+    //       context AND no singleton configured, it returns default (null).
     // ---------------------------------------------------------------------
     [Fact]
     public void DbBase_Find_with_null_context_returns_default()
@@ -238,6 +239,29 @@ public class BeforeTests
         Assert.Null(result);
     }
 
+    // (13b) When no explicit context is passed, Find falls back to the
+    //       DbBase.Context singleton instead of returning null.
+    [Fact]
+    public void DbBase_Find_falls_back_to_Context_singleton_when_db_is_null()
+    {
+        var db = new DbContext();
+        var acme = new Client { Name = "SingletonCo" };
+        db.Attach(acme);
+
+        DbBase.Context = db;
+        try
+        {
+            var result = DbBase.Find<Client>(c => c.Name == "SingletonCo", null);
+
+            Assert.NotNull(result);
+            Assert.Same(acme, result);
+        }
+        finally
+        {
+            DbBase.Context = null;   // cleanup
+        }
+    }
+
     // ---------------------------------------------------------------------
     // (14) Client.FindByName delegates to DbBase.Find and works correctly.
     // ---------------------------------------------------------------------
@@ -268,6 +292,28 @@ public class BeforeTests
         Assert.Null(result);
     }
 
+    // (15b) FindByName with no context argument uses the Context singleton.
+    [Fact]
+    public void Client_FindByName_uses_Context_singleton_when_db_omitted()
+    {
+        var db = new DbContext();
+        var acme = new Client { Name = "Acme Corp" };
+        db.Attach(acme);
+
+        DbBase.Context = db;
+        try
+        {
+            var result = Client.FindByName("Acme Corp");
+
+            Assert.NotNull(result);
+            Assert.Same(acme, result);
+        }
+        finally
+        {
+            DbBase.Context = null;   // cleanup
+        }
+    }
+
     // ---------------------------------------------------------------------
     // (16) The class-level DbBase.Context singleton can be used as the
     //       implicit context source for Find operations — the classic