refactor: remove Source property from IRelationship

Remove the redundant `Source` property from `IRelationship` and its
implementations. Since the relationship component is inherently
stored on the source entity, the `Source` field was unnecessary and
required manual synchronization during deserialization.

This change simplifies the relationship model, reduces memory usage,
and removes the need for reflection-based fixups in the serializer.

Game.Blackjack/Commands/HitCommand.cs

@@ -41,7 +41,7 @@ public struct HitCommand : ICommand
 
         // Remove from deck, add to hand.
         world.RemoveComponent<InDeck>(cardEntity);
-        world.AddComponent(cardEntity, new Holds { Source = cardEntity, Target = handEntity });
+        world.AddComponent(cardEntity, new Holds { Target = handEntity });
     }
 
     internal static Entity FindEntity<T>(World world, Entity singletonEntity)

Game.Blackjack/Commands/NewRoundCommand.cs

@@ -42,7 +42,7 @@ public struct NewRoundCommand : ICommand
             foreach (var card in cards)
             {
                 world.RemoveComponent<Holds>(card);
-                world.AddComponent(card, new InDeck { Source = card, Target = deckEntity });
+                world.AddComponent(card, new InDeck { Target = deckEntity });
             }
         }