Proposal: Add `thinfilm_weight` input to supporting BSDF nodes

In shading models such as `open_pbr_surface` and `gltf_pbr`, we currently end up duplicating each thin-film supporting BSDF node, in order to correctly handle the thin-film and iridescence weights that these shading models require:

https://github.com/AcademySoftwareFoundation/MaterialX/blob/main/libraries/bxdf/open_pbr_surface.mtlx
https://github.com/AcademySoftwareFoundation/MaterialX/blob/main/libraries/bxdf/gltf_pbr.mtlx

Would it make sense to add a `thinfilm_weight` input to each thin-film supporting BSDF node, so that this duplication would no longer be required?

This would likely provide a significant performance improvement to these shading models, especially in filtered importance sampling contexts where environment lighting must be separately sampled and summed for each of the duplicated BSDFs.

Start by reading libraries/bxdf/open_pbr_surface.mtlx and libraries/bxdf/gltf_pbr.mtlx, focusing on the duplicated thin-film supporting BSDF nodes and their weight handling. Compare the current graph structure and relevant MaterialX node definitions, then determine the required design and validation scope; the issue does not name tests or define final acceptance criteria.

Written by the indexing model from the issue text.