## About The Pull Request
Retypes the object, attaches it to the crate as a vis_overlay, and gives
it the correct vis_flag to share the plane.
This also means it now layers correctly on multiz, when previously it
would show up on the lowest level.
## Why It's Good For The Game
/obj/ is not meant to be bloated with stuff that can easily be an
effect. Also fixes a bug
## Changelog
🆑
fix: Mystery box items no longer layer on the lowest z_level when in
multiz
code: mystery_box_item is now a type of /obj/effect/abstract
/🆑
3256f65a9e