(Feat) Add build cost

Assets/GAME/Script/Building/BuildCost.cs

@@ -0,0 +1,15 @@
+using Ashwild.Inventory;
+
+namespace Ashwild.Building
+{
+    /// <summary>
+    /// One resource line of a buildable's price: an authored item and how many of it a single
+    /// placement consumes. Mirrors CraftingIngredient — plain authoring data, no behaviour.
+    /// </summary>
+    [System.Serializable]
+    public struct BuildCost
+    {
+        public ItemData item;
+        public int quantity;
+    }
+}