Add mouse wheel scaling for ModelViewers in the simulator

[Yong-yuan-X]

Summary

• Scale the ModelViewer under the mouse pointer with the wheel in simulator User mode.
• Apply uniform model scaling as a desktop equivalent of the existing two-hand pinch gesture.
• Normalize pixel-, line-, and page-based wheel deltas.
• Preserve browser scrolling when no scalable ModelViewer is targeted.
• Update the simulator instructions and manual to describe the new control.
• Add tests for model targeting, scaling direction, delta normalization, and wheel event handling.

Motivation

The simulator did not provide a mouse-based equivalent of the two-hand pinch gesture used to scale ModelViewer objects.

Mouse wheel input should change the targeted model's scale without moving the simulator camera or changing the user's viewport.

Testing

• npm run test — 58 test files and 493 tests passed on Node.js 24
• npm run lint
• npm run format:check
• npm run build
• cd docs && npm run build

Closes #428

[dli7319]

This actually moves the simulator camera forwards and backwards.

@ruofeidu Is this what you had in mind?
#428 sounds like it should scale up and scale down 3D models.

[ruofeidu]

Hi Yong-yuan,

For XR, user's viewport is fixed by device... so we likely don't want to mount mouse wheels to camera forwards and backwards, can you make it apply to two-hands-pinch-zoom in and zoom out? Thanks!

[ruofeidu]

Change mouse wheel to scale ModelViewers.

[Yong-yuan-X]

@ruofeidu Sry, I misunderstood the intended behavior.

I’ll update the implementation so the mouse wheel scales the ModelViewer under the pointer, matching the existing two-hand pinch scaling interaction. I’ll remove the camera movement and navmesh-related changes, update the tests and documentation, and push the revised implementation to this PR.

[dli7319]

Awesome, Thanks!