feat: integrate inventory with cursor system, scene input, and GameScript

- ActionState: add ITEM action (value 4) to enum and get_action_name()
- MainGame: right-click cycles through all 5 actions including ITEM;
  skips ITEM if nothing selected; clears selection when cycling away
- Scene: guard _unhandled_input() against active inventory overlay;
  handle ITEM action for world-wide item use; add give_item(),
  remove_item(), strip_items() helper methods
- SetPiece: handle ITEM action by calling scene's _use_item_on_setpiece()
- GameScript: add GiveItem and GiveItemDeferred script step classes
  for item acquisition during cutscenes

SetPiece_.gd

@@ -75,3 +75,9 @@ func _input(event):
 				ActionState.Action.TALK:
 					if talked.get_connections().size() > 0:
 						emit_signal("talked")
+				ActionState.Action.ITEM:
+					if InventoryManager.selected_item:
+						var item_id = InventoryManager.selected_item
+						var scene = get_node_or_null("/root/Node2D/SceneViewport/background")
+						if scene and scene is Scene:
+							scene._use_item_on_setpiece(item_id, self)