Resolves #8493

Changes:

Switched OBJ face parsing to use .at() for array access instead of bracket notation. Negative indices (like -1 for last vertex) now work per OBJ spec, positive indices still get converted from 1-based to 0-based.

Added test OBJ file with negative indices and tests confirming it loads and produces same geometry as positive index version.

PR Checklist

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