Add event publishing for inventory and combat interactions

GameCore/Events/InventoryItemChangedEvent.cs

@@ -0,0 +1,11 @@
+using GameCore.ECS;
+using GameCore.Events.Interfaces;
+
+namespace GameCore.Events;
+
+public readonly struct InventoryItemChangedEvent(Entity owner, string itemId, int newQuantity) : IEvent
+{
+    public readonly Entity Owner = owner;
+    public readonly string ItemId = itemId;
+    public readonly int NewQuantity = newQuantity;
+}