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/Systems/DeckSetupSystem.cs

@@ -36,7 +36,7 @@ public class DeckSetupSystem : ISystem
                 {
                     var cardEntity = world.CreateEntity();
                     world.AddComponent(cardEntity, new Card { Suit = suit, Rank = rank });
-                    world.AddComponent(cardEntity, new InDeck { Source = cardEntity, Target = deckEntity });
+                    world.AddComponent(cardEntity, new InDeck { Target = deckEntity });
                 }
             }
 
@@ -81,7 +81,7 @@ public class DeckSetupSystem : ISystem
         }
         for (int i = 0; i < cardEntities.Length; i++)
         {
-            world.AddComponent(cardEntities[i], new InDeck { Source = cardEntities[i], Target = deckEntity });
+            world.AddComponent(cardEntities[i], new InDeck { Target = deckEntity });
         }
     }
 }