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/CardEffects.cs

@@ -206,7 +206,7 @@ public class NunEffect : ICardEffect
             world.RemoveComponent<OnField>(target);
             if (world.HasComponent<Card>(target))
                 world.RemoveComponent<Card>(target);
-            world.AddComponent(target, new InDeck { Source = target, Target = discardDeck });
+            world.AddComponent(target, new InDeck { Target = discardDeck });
         }
         world.Commands.Enqueue(new DrawCardCommand { Player = pending.Player, Kind = null });
         world.RemoveSingleton<PendingNun>();
@@ -293,7 +293,7 @@ public class PegasusEffect : ICardEffect
             world.RemoveComponent<OnField>(target);
             if (world.HasComponent<Card>(target))
                 world.RemoveComponent<Card>(target);
-            world.AddComponent(target, new HeldBy { Source = target, Target = pending.Player });
+            world.AddComponent(target, new HeldBy { Target = pending.Player });
         }
         world.RemoveSingleton<PendingPegasus>();
         return true;
@@ -321,7 +321,7 @@ public class CurseMasterEffect : ICardEffect
             {
                 var skull = world.CreateEntity();
                 world.AddComponent(skull, new Skull());
-                world.AddComponent(skull, new PlacedOn { Source = skull, Target = pt });
+                world.AddComponent(skull, new PlacedOn { Target = pt });
             }
         }
         world.RemoveSingleton<PendingCurseMaster>();
@@ -366,14 +366,14 @@ public static class CardEffectHelpers
     {
         var horn = world.CreateEntity();
         world.AddComponent(horn, new Horn());
-        world.AddComponent(horn, new PlacedOn { Source = horn, Target = target });
+        world.AddComponent(horn, new PlacedOn { Target = target });
     }
 
     public static void AddSkull(World world, Entity target)
     {
         var skull = world.CreateEntity();
         world.AddComponent(skull, new Skull());
-        world.AddComponent(skull, new PlacedOn { Source = skull, Target = target });
+        world.AddComponent(skull, new PlacedOn { Target = target });
     }
 
     public static Entity GetBanner(World world, Entity player)

Game.CardWars/Commands/PlayCardCommand.cs

@@ -38,7 +38,7 @@ public struct PlayCardCommand : ICommand
 
         // Move from hand to field.
         world.RemoveComponent<HeldBy>(CardEntity);
-        world.AddComponent(CardEntity, new OnField { Source = CardEntity, Target = fieldOwner });
+        world.AddComponent(CardEntity, new OnField { Target = fieldOwner });
         world.AddComponent(CardEntity, new Card { Rank = Rank, FaceDown = FaceDown });
 
         // Resolve on-play effects via registry.

Game.CardWars/Components/HeldBy.cs

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

Game.CardWars/Components/InDeck.cs

@@ -9,6 +9,5 @@ namespace Game.CardWars;
 [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.CardWars/Components/OnField.cs

@@ -9,6 +9,5 @@ namespace Game.CardWars;
 [MessagePackObject]
 public record struct OnField : IRelationship
 {
-    [Key(0)] public Entity Source { get; set; }
-    [Key(1)] public Entity Target { get; set; }
+    [Key(0)] public Entity Target { get; set; }
 }

Game.CardWars/Components/PlacedOn.cs

@@ -9,6 +9,5 @@ namespace Game.CardWars;
 [MessagePackObject]
 public record struct PlacedOn : IRelationship
 {
-    [Key(0)] public Entity Source { get; set; }
-    [Key(1)] public Entity Target { get; set; }
+    [Key(0)] public Entity Target { get; set; }
 }

Game.CardWars/GameUtil.cs

@@ -38,7 +38,7 @@ public static class GameUtil
 
         var cardEntity = cards.First();
         world.RemoveComponent<InDeck>(cardEntity);
-        world.AddComponent(cardEntity, new HeldBy { Source = cardEntity, Target = player });
+        world.AddComponent(cardEntity, new HeldBy { Target = player });
         return true;
     }
 
@@ -54,7 +54,7 @@ public static class GameUtil
         for (int i = cards.Length - 1; i >= 0; i--)
             world.RemoveComponent<InDeck>(cards[i]);
         for (int i = 0; i < cards.Length; i++)
-            world.AddComponent(cards[i], new InDeck { Source = cards[i], Target = deck });
+            world.AddComponent(cards[i], new InDeck { Target = deck });
     }
 
     /// <summary>Get all field cards for a player.</summary>
@@ -135,7 +135,7 @@ public static class GameUtil
         {
             world.RemoveComponent<OnField>(card);
             world.RemoveComponent<Card>(card);
-            world.AddComponent(card, new InDeck { Source = card, Target = discardDeck });
+            world.AddComponent(card, new InDeck { Target = discardDeck });
         }
 
         // Destroy horns and skulls placed on discarded cards.
@@ -156,7 +156,7 @@ public static class GameUtil
         foreach (var card in cards)
         {
             world.RemoveComponent<InDeck>(card);
-            world.AddComponent(card, new InDeck { Source = card, Target = drawDeck });
+            world.AddComponent(card, new InDeck { Target = drawDeck });
         }
     }
 }

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);
             }