Test/houdini22 macos clean build (will be split into smaller PRs)

[gripeyes]

Compatibility:

minor

Issues/Tickets:

• [Improve Houdini integration and documentation — OpenMoonRay/openmoonray#265](Improve Houdini integration and documentation #265)
• [Houdini integration discussion — OpenMoonRay/openmoonray#270](New HdMoonray version #270)
• [Apple Silicon half-float framebuffer fixes — OpenMoonRay/openmoonray#272](Pin Apple Silicon half conversion fixes #272)
• [Houdini dependency alignment — OpenMoonRay/openmoonray#273](Align dependency packages with Houdini 21 #273)
• [Metal GPU accelerator atomic update — OpenMoonRay/openmoonray#274](Pin Metal GPU accelerator atomic update #274)
• [Original Houdini DCC integration work — OpenMoonRay/moonray_dcc_plugins#3](Houdini 20.5: expose MoonRay Solaris material builder moonray_dcc_plugins#3)
• [Updated DCC plugin and HDA generation base — OpenMoonRay/moonray_dcc_plugins#5](Update hdas for openmoonray 2026.29.1 moonray_dcc_plugins#5)

Release notes comment:

Adds experimental macOS build and Solaris integration support for Houdini 22, USD 26.05, Python 3.13, and MaterialX.

Comments for the reviewer:

This is an umbrella draft PR documenting the complete Houdini 22 macOS integration and validation effort. It is not intended to merge as one large change. The completed work will be separated into smaller focused PRs after the overall integration has been reviewed.

The branch covers:

• Houdini 22.0.408, USD 26.05, Python 3.13, and C++20 alignment.
• Updated macOS third-party dependencies and build configuration.
• Hdm10 and USD 26 compatibility.
• MaterialX shader building and HDA generation.
• Solaris IPR lifecycle and render-settings updates.
• Apple Silicon half-float framebuffer and AOV fixes.
• MoonRay light filters, mesh tessellation controls, native material builder, and native SpotLight controls.
• Coordinated hdMoonray, moonray, scene_rdl2, SDR, MaterialX, Moonshine, Arras, and DCC plugin revisions.

The intermediate root submodule-pin commits are intentionally retained as validation checkpoints. They document which revisions were tested during the investigation, including unsuccessful lifecycle experiments that were later reverted or replaced by the final clean submodule branches.

A clean Release and Debug build was completed on Apple Silicon using Houdini 22.0.408. Solaris renderer discovery, interactive updates, MaterialX loading, HDA generation, render settings, light updates, and framebuffer output were exercised.

The smaller PRs will be prepared from clean upstream bases and will contain only their final focused changes.

Full build, dependency, submodule, launch, and validation details are available in the [Houdini 22 macOS build and validation notes](https://github.com/gripeyes/openmoonray/blob/test/houdini22-clean-build/building/macOS/Houdini22_build_notes.md).

Look or scene setup change:

No intentional look changes are expected. The Apple Silicon framebuffer and AOV corrections may change previously incorrect, empty, or invalid half-float outputs.

Special notes for production:

This is an experimental integration draft and is not currently intended as a production release.

The current validation configuration is pinned to Houdini 22.0.408. Qt applications and the MoonRay GUI were excluded, and Metal rendering remains disabled in the tested preset.

Attention/Reviewers:

AI Assisted Development:

Assisted-by: OpenAI Codex / GPT-5.6

Checklist:

• Documentation has been updated: [Houdini 22 macOS build and validation notes](https://github.com/gripeyes/openmoonray/blob/test/houdini22-clean-build/building/macOS/Houdini22_build_notes.md)
• Includes new unit tests.
• Includes new RATS tests.