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 });
             }
         }
 

Game.Blackjack/Components/Holds.cs

@@ -10,6 +10,5 @@ namespace Game.Blackjack;
 [MessagePackObject]
 public record struct Holds : IRelationship
 {
-    [Key(0)] public Entity Source { get; set; }
-    [Key(1)] public Entity Target { get; set; }
+    [Key(0)] public Entity Target { get; set; }
 }

Game.Blackjack/Components/InDeck.cs

@@ -10,6 +10,5 @@ namespace Game.Blackjack;
 [MessagePackObject]
 public record struct InDeck : IRelationship
 {
-    [Key(0)] public Entity Source { get; set; }
-    [Key(1)] public Entity Target { get; set; }
+    [Key(0)] public Entity Target { get; set; }
 }

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 });
         }
     }
 }