Fix UV shell connectivity when importing non-indexed faceVarying UVs

[paulgolter]

Problem

When a mesh has a faceVarying UV primvar without authored indices (primvars:st but no primvars:st:indices — common for USD written by other DCCs), the importer creates one Maya UV per face vertex with no sharing.

The imported UV values themselves are correct, but no UVs are shared between neighboring faces, so every face becomes its own UV shell. A cube that should import with 14 UVs in a single shell instead imports with 24 UVs in 6 shells, and the layout falls apart into individual faces as soon as the UVs are edited.

The same data imports correctly when authored with indices, so whether an asset arrived intact depended on how the exporting application chose to write its UVs.

Fix

In assignUVSetPrimvarToMesh, when a faceVarying UV primvar has no authored indices, weld values that are equal on the same mesh vertex into a single shared UV and synthesize the assignment indices. Welding per vertex restores connectivity between adjacent faces without fusing UV islands that merely overlap in UV space (e.g. stacked/mirrored shells). The result is identical to what an indexed export of the same data would produce.

Indexed primvars, other interpolations, and the left-handed winding reversal are untouched.

Tests

Added a right-handed and a left-handed cube with non-indexed faceVarying UVs to the UsdImportUVSetsTest fixtures (both variants), with new cases in testUsdImportUVSets.py asserting values, UV count (14), and shell count (1). All existing UV set, mesh, and color set import tests pass.