Improvements for GLTF loading and scene registry

[korvixhq]

What does this PR do?

Performance improvements for GLTF loading and scene registry:

• Added useCachedGLTF hook that caches loaded GLTFs by URL, eliminating redundant network requests when multiple items use the same asset.
• Removed redundant scene.traverse() call per item mount (previously ran once per item even with identical assets)
• Fixed registry cleanup null check for edge cases

How to test

1. Open network tab and load a scene with multiple items of the same type (e.g., 10+ identical chairs)
2. Verify only 1 network request is made per unique GLTF asset
3. Place, move, and delete items to ensure no crashes or visual glitches occur with the cached GLTF and registry cleanup

Screenshots / recordings

n/a

Checklist

• I've tested this locally with bun dev
• My code follows the existing code style (run bun check to verify)
• I've updated relevant documentation (if applicable)
• This PR targets the main branch