Fix ListPrims returning invalid path information

[Copilot]

ListPrims was returning Prims with <#INVALID#> paths because TraverseRec used prim.local_path() (reserved for composition, often unset) instead of the computed absolute path.

Changes

• src/tydra/scene-access.cc: Updated TraverseRec and TraverseShaderRec to:

  • Use prim.absolute_path() when available (set by Stage::compute_absolute_prim_path_and_assign_prim_id())
  • Fall back to constructing path from prim.element_name() otherwise

• tests/unit/unit-listprims.{h,cc}: Added unit test covering both scenarios (with/without computed absolute paths)

Example

// Before fix: returns <#INVALID#> paths
MeshMap meshes;
tinyusdz::tydra::ListPrims(stage, meshes);
for (const auto &mesh : meshes) {
    std::cout << mesh.first << "\n";  // Was: /#INVALID#/#INVALID#
}

// After fix: returns correct paths
// Output: /root/mesh0

The fix maintains backward compatibility for both file-loaded Stages (where absolute paths are computed automatically) and programmatically created Stages (where they may not be).

Original prompt

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This section details on the original issue you should resolve

<issue_title>ListPrims not populating path information</issue_title>
<issue_description>Describe the bug
When calling ListPrims the prims returned all seem to have an <#INVALID#> path (absolute_path, local_path & element_path)

To Reproduce
Steps to reproduce the behavior:

  MeshMap meshes;
  tinyusdz::tydra::ListPrims(stage, meshes);
  for (const auto &mesh : meshes) {
    const tinyusdz::Prim &prim = *(mesh.second);
    std::cout << prim.element_name() << " : " << prim.absolute_path() << "\n";
  }  

Expected behavior
I'd expect to be able to access the path information.

But, perhaps I'm using it wrong?

Desktop (please complete the following information):

• OS: Windows
• latest build
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes lighttransport/tinyusdz#263

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