Box3D Example

[alkavan]

Based on Bullet and Jolt examples, but replaced some OOPy stuff with more simple variation, introducing light weight Body data structure.

• Most debug options actually work.
• Using FetchContent_ for Box3D — as suggested in the box3d repository.
• Some of the debug static callbacks, very specific parts of the example, and some docs were generated with the latest Grok (4.5). However, no agent/harness or some kind of automated process was used.

Ready for easy conversion to proper integration, then Box3DIntegration/ can be removed from example — but I would prefer merging the example first to allow people experiment, and wait for a more stable Box3D release to integrate, although 0.1.0 has no issues as far as I'm aware of.