Fix WebGPU XR stereo viewport and harden vr-lod example

[mvaligursky]

Fix WebGPU XR stereo rendering and polish the VR LOD example so it runs reliably on Apple Vision Pro.

Changes:

• Renderer#setCameraUniforms and GSplatManager#sortGpuHybrid now read the per-eye viewport from camera.xr.views.list[0].viewport directly instead of inferring per-eye width by halving target.width. The XR bridge already reports the correct per-eye rectangle for both side-by-side single-texture layouts and multi-pass per-eye-view layouts, so the explicit halving (which produced quarter-eye widths under WebGPU multiview) is gone.
• vr-lod example: drop the desktop mirror to half-DPR when devicePixelRatio >= 2. The previous 2× DPR triggered Vision Pro Safari to reboot after ~15 s of gsplat fragment load.
• vr-lod example: replace the controls-panel Enter VR button (which could not provide WebXR transient activation across the iframe boundary) with a fixed-position DOM button rendered inside the example document. The button hides while a session is active and reappears after the session ends. Remove the now-redundant XR panel from the controls.