Add native OBJ + MTL + texture loader for custom objects

[Ishan-Banjara]

Summary

Extends the "Custom object..." loader to natively support .obj files with optional .mtl material files and texture images, alongside existing .stl support.

🔗 Live demo: https://ishan-banjara.github.io/Rocksi/

Motivation

The original custom object loader only supports .stl files, which lack material and color information. Many common 3D models (especially from Free3D, Sketchfab, etc.) ship as .obj + .mtl + textures. This PR lets users load those directly with full visual fidelity.

Changes

All changes contained in src/simulator/objects/createObjects.js:

• Added OBJLoader and MTLLoader imports from three/examples/jsm/loaders/
• Renamed loadUserSTL() → loadUserFile() as a format router
• File picker now accepts multi-file selection for .obj + .mtl + textures
• Added loadUserOBJ() with LoadingManager.setURLModifier for blob URL resolution of texture references
• Added attachOBJToSimObject() which:
  • Preserves MTL colors/textures (doesn't override with block's colour field)
  • Converts materials to MeshPhongMaterial for compatibility with Rocksi's highlight system (prevents a material.emissive crash)
  • Auto-scales model to visible size
  • Centers XY, rests base on floor

The existing STL path is completely unchanged; backward compatible.

How to Test

1. Drag "Place an object" block into workspace
2. Select "Custom object..."
3. Multi-select .obj + .mtl + texture images in the picker
4. Model appears with full materials

Or try directly at the live demo above.

Notes

• No new runtime dependencies (OBJLoader/MTLLoader already ship with three)
• README.md has a small fork-description added; happy to revert that if preferred; the substantive change is entirely in createObjects.js