Basic scene graph

Implement a basic scene graph structure for describing hierarchical 3D scenes (i.e., a stack of transformations and data on the leaves). This structure is not intended to describe rendering at all, it is just a data structure for 3D scenes that can later be used by a scene renderer.

No files, tests, or entry points are named. Start by surveying the C++ project structure and its existing 3D or rendering abstractions; done means a basic hierarchical scene data structure exists without rendering behavior and can represent transformation stacks with data on leaf nodes.

Written by the indexing model from the issue text.