Add an `aboveStackSlot` prop to PromptBox3D rendered as the first
child of its `bottom-4 flex-col` stack, and thread a
`promptboxAboveStackSlot` prop through Stage3D / Stage3DBody. The lib
now owns the vertical math, so the webapp's button never overlaps the
prompt input regardless of textarea height — replaces the host-side
ResizeObserver/MutationObserver positioning that was unreliable in
production builds.
bflatastic
87153de16d