fix(corner_radius): check LayerHookId for layer surfaces

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.

src/wayland/protocols/corner_radius.rs

@@ -190,7 +190,7 @@ where
                     let radius_exists = with_states(surface.wl_surface(), |surface_data| {
                         let hook_id = surface_data
                             .data_map
-                            .get_or_insert_threadsafe(|| ToplevelHookId::new(None));
+                            .get_or_insert_threadsafe(|| LayerHookId::new(None));
                         let guard = hook_id.lock().unwrap();
                         guard.as_ref().map(|(_, t)| t.upgrade().is_ok())
                     });