Add MaterialX validation script and CI workflow

[jamportz]

Summary

• Adds validation/validate_mtlx.py, a script that validates .mtlx files against the MaterialX standard libraries
• Adds .github/workflows/validate-mtlx.yml, which runs the validator on reference/open_pbr_surface.mtlx and all examples/*.mtlx on every PR or push that touches .mtlx files

How it works

The validator uses pip install materialx (no build-from-source required), loads the standard libraries, and runs doc.validate(). It also performs an explicit nodename reference check on the standalone document — this catches dangling connections that the with-libraries validation misses when a file shares a name with a standard library file (as open_pbr_surface.mtlx does with libraries/bxdf/open_pbr_surface.mtlx).

Test plan

• Workflow triggers on PRs that modify .mtlx files
• reference/open_pbr_surface.mtlx passes
• All examples/*.mtlx pass
• A PR that introduces a broken nodename reference fails CI

🤖 Generated with Claude Code

[najibna]

The explicit nodename check plus check_library_shadowing() is the part that actually matters — reference/open_pbr_surface.mtlx shares a basename with the MaterialX stdlib copy, so doc.validate() alone can miss dangling connections.

validation/validate_mtlx.py is a large general linter, and pip install materialx is unpinned. --version is also a store_true next to a required positional file, so python validation/validate_mtlx.py --version fails. A thin wrapper around doc.validate() plus the shadowing check would be easier to maintain.