USDZExporter: Fix AR anchoring on root prim

[arielelkin]

Related issues:

• USDZ exporter with target wall placement #21995
• ar-placement ignored on quick look google/model-viewer#3989

Description

USDZExporter currently writes preliminary:anchoring:type and preliminary:planeAnchoring:alignment as bare token properties on the nested /Root/Scenes/Scene prim. Apple documents Preliminary_AnchoringAPI as an applied API schema and requires anchorable prims to be defined at the document root. As a result, Quick Look may ignore the requested alignment and fall back to horizontal placement.

This change applies Preliminary_AnchoringAPI to the root-level /Root model prim and authors both anchoring properties there as uniform token values. It also removes the legacy /Root/Scenes/Scene scene-library wrapper and authors exported scene content directly beneath /Root. The existing ar options, defaults, and USDZ packaging remain unchanged. When includeAnchoringProperties is false, the schema and both properties continue to be omitted without accessing options.ar.

The misc_exporter_usdz example now provides option to render a Bike (for horizontal anchoring) model and a Plaque (for vertical anchoring), with corresponding buttons and a “View in AR” action. Selecting a model updates both the Three.js preview and the USDZ targeted by the Quick Look link. AR launch is enabled only in Safari on iPhone and iPad; other browsers display a disabled availability message. Generated blobs use Quick Look’s model/vnd.usdz+zip MIME type.

Prior implementation context:

• USDZExporter: Add anchoring support. #22854
• USDZExporter: Make including anchoring properties optional. #28363

Apple schema references:

• https://developer.apple.com/documentation/usd/preliminary-anchoringapi
• https://developer.apple.com/documentation/usd/placing-a-prim-in-the-real-world

Tested with:

npm test

[github-actions]

🖼️ E2E screenshot tests

✅ All examples render correctly again (run).

[alexwakeman]

This looks really useful - be great if maintainers could take a look