USDZExporter: Refactor to use two stages

[mkeblx]

Description

This is a refactoring of the exporter to process in two stages: first building up trees of nodes, and then doing the final stringifying (and with the same final output as before, modulo whitespace which is improved). It stops short of fully breaking down properties and leaves most as strings.

This has the significant benefits of making upcoming changes much simpler (like object hierarchy (#31235), just change the tree layout), and in general easier to maintain and extend.

[Copilot]

Pull Request Overview

This PR refactors the USDZExporter to use a two-stage process: first building a tree of USD nodes, then converting that tree to final USD string output. The refactoring maintains the same final output while improving code maintainability and making future changes easier.

• Introduces a new USDNode class to represent USD nodes with properties, metadata, and children
• Refactors material, mesh, and camera building functions to create node trees instead of direct string output
• Consolidates string generation into the USDNode.toString() method for consistent formatting

Comments suppressed due to low confidence (2)

examples/jsm/exporters/USDZExporter.js:679

• [nitpick] The function name 'buildTextureNodes' is inconsistent with the existing naming pattern. Other functions in the file use 'build' prefix for single objects (buildMaterial, buildCamera, buildMeshObject). Consider renaming to 'buildTextureNodeSet' or 'createTextureNodes' to better reflect that it returns multiple nodes.

	function buildTextureNodes( texture, mapType, color ) {

examples/jsm/exporters/USDZExporter.js:507

• [nitpick] The function name 'buildMeshNode' is inconsistent with the original 'buildMesh' function it replaces. Since this is a refactoring, consider keeping the original name 'buildMesh' to maintain consistency with other build functions in the codebase.

function buildMeshNode( geometry ) {

[mrdoob]

Anything from the copilot review that seems worth doing?

[mkeblx]

Anything from the copilot review that seems worth doing?

Yes, the metadata handling suggestion makes sense. I simplified in a similar way.

[Mugen87]

For testing:

https://rawcdn.githack.com/mkeblx/three.js/usdz-exp-refactor/examples/misc_exporter_usdz.html
https://rawcdn.githack.com/mkeblx/three.js/usdz-exp-refactor/editor/index.html