base_color 0:3, metallic 3:4, roughness 4:5, alpha 5:6 - the pipeline's own
pbr_attr_layout. o_voxel's baker indexes this dict BY NAME and raises KeyError deep
inside to_glb on any missing slot rather than at the call, so a partial layout looks
like a baker bug. Needed by the texture stage; recorded now while it is in hand.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>