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

ActionState.gd

@@ -1,6 +1,6 @@
 extends Node
 
-enum Action { WALK = 0, LOOK = 1, TOUCH = 2, TALK = 3 }
+enum Action { WALK = 0, LOOK = 1, TOUCH = 2, TALK = 3, ITEM = 4 }
 
 var current_action: int = Action.WALK:
 	set(value):
@@ -22,6 +22,7 @@ func get_action_name() -> String:
 		Action.LOOK: return "look"
 		Action.TOUCH: return "touch"
 		Action.TALK: return "talk"
+		Action.ITEM: return "item"
 	return "walk"
 
 func get_action_enum() -> int: