fix: preserve vertex attributes with smooth normals - #9233

Resolves #9205

Changes:

• Fixed computeNormals(SMOOTH) so it no longer permanently deduplicates geometry vertices based only on position.
• Smooth normals are calculated using temporary position-based vertex groups and then mapped back to the original vertices.
• This preserves per-vertex attributes such as UVs while still allowing vertices at the same position to share smooth normals.
• Added a unit test covering vertices with the same position but different UV coordinates.

Screenshots of the change:

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated