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/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; }
 }