GetCornerRadiusLayer looked up the existing hook via ToplevelHookId while
inserting into LayerHookId a few lines later. Both are
Mutex<Option<(HookId, Weak<..>)>> so it compiles, but they are distinct
data_map slots: the "does this surface already have a corner-radius object?"
check for a layer surface read the toplevel slot, so it could not see a layer
object that was already there, and it created a toplevel entry on a layer
surface as a side effect.
Every other read of the layer hook already uses LayerHookId.
Disclosure: found and written with AI assistance (Claude). The change is the
one line quoted above; I understand it and verified the tree builds with it.
