pub fn add_slice_plane_auto() -> SlicePlaneHandleExpand description
Adds a slice plane with an auto-generated name like “Scene Slice Plane 0”.
Mirrors C++ Polyscope’s addSlicePlane() (no-args overload). Returns a
handle to the newly created plane. The chosen index is the smallest
non-negative integer N for which “Scene Slice Plane N” is not already in
use, so removing a middle plane and re-adding will reclaim that index.
Search and creation happen under a single with_context_mut lock — without
that, two concurrent callers could pick the same name and one would receive
a handle to the other’s plane (since core’s add_slice_plane is an upsert).