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

MainGame.gd

@@ -78,5 +78,15 @@ func _input(event):
 	if event.is_action_released("quit"):
 		get_tree().quit()
 	if event.is_action_released("right_click") and not is_cursor_locked:
-		ActionState.current_action = (ActionState.current_action + 1) % 4
-		Input.set_custom_mouse_cursor(cursors[ActionState.current_action], Input.CursorShape.CURSOR_ARROW, Vector2(0,0))
+		var prev_action = ActionState.current_action
+		ActionState.current_action = (ActionState.current_action + 1) % 5
+		if ActionState.current_action == ActionState.Action.ITEM:
+			if InventoryManager.selected_item:
+				Input.set_custom_mouse_cursor(cursors[ActionState.current_action], Input.CursorShape.CURSOR_ARROW, Vector2(0,0))
+			else:
+				ActionState.current_action = (ActionState.current_action + 1) % 5
+				Input.set_custom_mouse_cursor(cursors[ActionState.current_action], Input.CursorShape.CURSOR_ARROW, Vector2(0,0))
+		else:
+			if prev_action == ActionState.Action.ITEM:
+				InventoryManager.clear_selection()
+			Input.set_custom_mouse_cursor(cursors[ActionState.current_action], Input.CursorShape.CURSOR_ARROW, Vector2(0,0))