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.CardWars/Systems/GameSetupSystem.cs

@@ -57,7 +57,7 @@ public class GameSetupSystem : ISystem
         var defs = CardDataLoader.LoadDefinitions(world, _cardDataCsv);
         foreach (var (defEntity, _) in defs)
         {
-            world.AddComponent(defEntity, new InDeck { Source = defEntity, Target = publicDeck });
+            world.AddComponent(defEntity, new InDeck { Target = publicDeck });
         }
 
         // Shuffle public deck.
@@ -79,12 +79,12 @@ public class GameSetupSystem : ISystem
             world.AddComponent(leader, new Leader());
             world.AddComponent(leader, new CardDef { Name = $"领袖{i}", Ranks = new[] { 0 }, Effect = CardEffect.None, Kind = CardKind.Faction });
             world.AddComponent(leader, new Card { Rank = 0, FaceDown = false });
-            world.AddComponent(leader, new OnField { Source = leader, Target = player });
+            world.AddComponent(leader, new OnField { Target = player });
 
             // Create banner.
             var banner = world.CreateEntity();
             world.AddComponent(banner, new Banner());
-            world.AddComponent(banner, new OnField { Source = banner, Target = player });
+            world.AddComponent(banner, new OnField { Target = player });
 
             // Create faction deck.
             var factionDeck = world.CreateEntity();

Game.CardWars/Systems/ScoringSystem.cs

@@ -42,7 +42,7 @@ public class ScoringSystem : ISystem
             if (castles.Count > 0)
             {
                 var castle = castles[0];
-                world.AddComponent(castle, new HeldBy { Source = castle, Target = winner });
+                world.AddComponent(castle, new HeldBy { Target = winner });
                 world.RemoveComponent<Castle>(castle);
             }