Optimize subset index grouping by material

[ryanngit]

Summary

• group requested subset indices by material in one pass before rebuilding subset geometry
• keep the existing per-material insertion path so group ordering and counts stay compatible
• add a focused regression test for the material grouping behavior

Verification

• npm test -- --runInBand test/IFC/Components/SubsetUtils.test.ts
• npm run build-declaration

Fixes #110