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.

src/Before/Client.cs

@@ -25,16 +25,20 @@ public class Client : DbBase
     }
 
     /// <summary>
-    /// Convenience lookup: walks the <paramref name="db"/>'s tracked entities,
-    /// finds the first <see cref="Client"/> whose <see cref="Name"/>
-    /// equals <paramref name="name"/>.  Returns <c>null</c> when no match.
+    /// Convenience lookup: finds the first <see cref="Client"/> whose
+    /// <see cref="Name"/> equals <paramref name="name"/>.  Returns
+    /// <c>null</c> when no match.
     ///
     /// This mirrors how an ActiveRecord-style ORM might surface a static
-    /// finder on the domain class itself — it rides the <c>DbContext</c> leak
-    /// from the entity's back-reference. Delegates to
+    /// finder on the domain class itself — it rides the <c>DbContext</c>
+    /// leak from the entity's back-reference. Delegates to
     /// <see cref="DbBase.Find{T}"/> for the common traversal logic.
+    ///
+    /// The context is optional: when omitted, the <see cref="DbBase.Context"/>
+    /// singleton is used, so call sites read like
+    /// <c>Client.FindByName("Jane Doe")</c> — no context threading required.
     /// </summary>
-    public static Client? FindByName(string name, DbContext? db)
+    public static Client? FindByName(string name, DbContext? db = null)
         => DbBase.Find<Client>(c => c.Name == name, db);
 }