PBR Next

As the number of possible combinations of new PBR extensions grows, we should provide a set of models that test most (if not all) possible permutations.

- MR, sheen, clearcoat, transmission, ...
- Factors & texture slots
- Reused images

No files, tests, or entry points are named. Start by locating the asset-generation entry points and existing models, then define a representative set covering the listed PBR extensions, factors, texture slots, and reused images; completion means the permutation coverage is documented and the generated assets validate.

Written by the indexing model from the issue text.