Function createHierarchyInstancePool

Build a fixed-capacity thin-instance pool from a template hierarchy.

Call before registerScene(), after the hierarchy's parent links are established. Fresh loadGltf() hierarchies are also supported: this helper materializes child parent links from the children arrays before snapshotting template world matrices.

The template meshes are converted to thin-instanced meshes with an active count of zero, so they do not draw until instances are added.