Sort a sphere BVH with split planes

from blog Computer Graphics & Programming with Pekka Väänänen - 30fps.net, | ↗ original
Hierarchical frustum culling is quick to do with a bounding volume hierarchy (BVH). Possibly the simplest of such is the sphere BVH where each node is represented by a sphere that is large enough to hold all of its children’s bounding spheres: