SFC reordering

In my FerriteCon 2025 talk I have shown how to use Gecko orderings to boos assembly performance (see github.com/termi-official/ferritecon2025-graph-ordering for code and https://www.youtube.com/watch?v=jkCp3q5UOkw for a recording of the talk if more details are needed). However, Gecko solves a full graph linear arrangement problem. A cheaper alternative is to use the use the cell centers of each element, put an quad/octree (in 2D/3D) over this set of points until each leaf only has at most a single node. We then reorder the elements to follow the SFC of the octree (morton order/hilbert order/...). This way we also preserve locality of the elements. The same can be done with the nodes.