Validator - check empty child within parent

@Rudraksha20

From the roadmap:

> When a tile has no content (because it is just used for culling), its boundingVolume must be completely inside its parent tile's boundingVolume.

Start by locating the validator's existing tile and bounding-volume validation entry points, then trace how an empty child tile is identified. Implement the roadmap requirement that its boundingVolume be completely inside the parent boundingVolume, and add or update a focused validation test covering this case.

Written by the indexing model from the issue text.