Feature: per-tower construction-phase build bisuals

Adding the correct pipeline for creating the different building phases for towers. Adding a default set with primitive cubes.

Assets/_Project/Scripts/Towers/TowerDefinition.cs

@@ -68,6 +68,11 @@ namespace TD.Towers
                  "replace with a real mesh when art is available.")]
         public GameObject TowerPrefab;
 
+        [Tooltip("Optional per-tower construction-stage visuals shown while this tower is " +
+                 "being built. Leave empty to use the project-default set assigned on the " +
+                 "BuildSiteVisual prefab.")]
+        public ConstructionPhaseSet ConstructionPhases;
+
         [Tooltip("Icon shown on this tower's tile in the HUD build menu. Auto-generated from " +
                  "the TowerPrefab by the 'TD/Generate Tower Icons' menu item, or assign a " +
                  "Sprite manually. Null falls back to the grey placeholder icon.")]