OpenUSD Material

Author materials using UsdShadeMaterial and PBR UsdPreviewSurface inputs, then bind them to prims. Requires the pxr OpenUSD runtime — all tools fail with a clear error when pxr is not available.

Workflow

1. Ensure a stage exists (use openusd-stage__create_stage or openusd-project).
2. create_material — define a UsdShadeMaterial prim.
3. create_preview_surface — attach a UsdPreviewSurface shader to the material.
4. bind_material — bind the material onto a geometry/shading prim.