Function loadLodGroupNode

Load an lod_group node on initial scene construction.

Pattern:

1. Create a THREE.Group for the lod_group; apply transform.
2. Recurse each child through loadSceneNodes so its geometry-specific loader runs and a placeholder mesh attaches.
3. After each child loads, locate its THREE node by name and record its coverage_fraction / position_bounds.
4. Register a LODGroupEntry with the registry — the entry controls per-child visibility on subsequent frames.