3MFLoader: Support external component parts.

[jameszhong2008]

Summary

Add support for 3MF component references that point to external model parts via p:path / path.

This fixes 3MF files exported by tools like BambuStudio where the main 3D/3dmodel.model contains composite objects whose meshes live in 3D/Objects/*.model.

Changes

• parse and preserve component part paths
• track model part path for each parsed model document
• resolve composite components against the referenced model part instead of always using the current model
• scope built object cache keys by modelPath + objectId to avoid collisions across external model parts
• throw clearer errors when referenced model parts or object resources are missing

Problem

The loader previously assumed every component.objectid referenced an object inside the current model part. For production-extension style 3MF files, components may instead reference objects stored in another model part such as 3D/Objects/object_1.model.

That caused resources.object[ objectId ] to resolve to undefined, eventually throwing when reading mesh.

Result

3MF files with external component parts can now be loaded correctly instead of failing during composite object construction.

[Mugen87]

Can you please share a test asset so it's possible to verify your change? The asset should be added to the drop-down UI field in https://threejs.org/examples/webgl_loader_3mf.

[Mugen87]

While reviewing your PR, I have noticed your branch jameszhong2008:fix-3mf-production-components is 4559 commits behind dev. That is not good. Please do a rebase.