feat(oecs): add support for reordering relationship sources

Introduces `RelationshipReordered` change kind and `ReorderSources`
method to allow reordering the source set of a relationship without
removing and re-adding components. This optimizes shuffling operations
like deck setup in Blackjack.

Game.Blackjack/Systems/DeckSetupSystem.cs

@@ -60,27 +60,10 @@ public class DeckSetupSystem : ISystem
         for (int i = cardEntities.Length - 1; i > 0; i--)
         {
             int j = Mulberry32.NextInt(ref seed, 0, i);
-
-            // Swap the InDeck relationship targets (cards are always in the deck,
-            // so there's nothing to swap except the cards themselves — but we
-            // shuffle the card order conceptually by removing and re-adding
-            // InDeck components in shuffled order). Actually, since InDeck
-            // is just a tag, we just re-shuffle the order in the source collection.
-            // The simplest approach: no need to swap component data; we just
-            // need to ensure the cards are iterated in shuffled order.
-            // We'll swap the card entities in the array.
             (cardEntities[i], cardEntities[j]) = (cardEntities[j], cardEntities[i]);
         }
 
-        // Now remove all InDeck and re-add in shuffled order so GetSources
-        // returns them in shuffled order.
-        for (int i = cardEntities.Length - 1; i >= 0; i--)
-        {
-            world.RemoveComponent<InDeck>(cardEntities[i]);
-        }
-        for (int i = 0; i < cardEntities.Length; i++)
-        {
-            world.AddComponent(cardEntities[i], new InDeck { Target = deckEntity });
-        }
+        // Reorder the source set in-place — no component add/remove needed.
+        world.ReorderSources<InDeck>(deckEntity, cardEntities);
     }
 }