Add Houdini 22 macOS MaterialX shader compatibility

[gripeyes]

Compatibility:

patch

Issues/Tickets:

OpenMoonRay/openmoonray#276
OpenMoonRay/openmoonray#275

Release notes comment:

Added Houdini 22 macOS compatibility for building and generating MaterialX shaders.

Comments for the reviewer:

This extracts the MaterialX shader source changes from the Houdini 22 macOS validation branch.

The changes:

• use Darwin-compatible linker options instead of Linux RUNPATH flags on macOS
• qualify standard and scene_rdl2 math functions for newer AppleClang/C++ builds
• invoke the shader-generation script through the configured CMake Python interpreter
• preserve the existing shader calculations and generated outputs

This does not duplicate OpenMoonRay/openmoonray#275. That PR fixes the ordering of scene_rdl2 and materialx_shaders in the complete OpenMoonRay build; this PR fixes compatibility inside the materialx_shaders repository itself.

The two historical validation commits were consolidated into one signed commit, and unrelated whitespace changes were removed.

Umbrella validation PR:
OpenMoonRay/openmoonray#276

Houdini 22 build notes:
https://github.com/gripeyes/openmoonray/blob/test/houdini22-clean-build/building/macOS/Houdini22_build_notes.md

Look or scene setup change:

No expected look change.

Special notes for production:

Attention/Reviewers:

AI Assisted Development:

Assisted-by: OpenAI Codex / GPT-5.6

Checklist:

• Documentation has been updated.
• Includes new unit tests.
• Includes new RATS tests.