TODO (cosmetic): Swap cube walls with cube slide joint limits + ball joint

[SimpleGeometry]

WRT binpick EEF: currently the cube is being prevented from moving too far via walls that only collide with the cube. But cube walls are an ugly workaround right now, especially as they are opaque.

Instead, if we replace the cube freejoint with 3 slide joints and a ball joint, we keep 6DoFs while being able to constrain where the cube goes via slide joint limits. See: google/brax#545