Add attribute system with core stats and gameplay components

GameCore/ECS/Entity.cs

@@ -0,0 +1,11 @@
+namespace GameCore.ECS;
+
+/// <summary>
+/// Represents a unique object in the game world.
+/// It's a simple struct containing an ID to keep it lightweight.
+/// All data associated with an Entity is stored in Components.
+/// </summary>
+public readonly struct Entity(int id)
+{
+    public readonly int Id = id;
+}