Create a parallel WebGPU XR path that uses XRGPUBinding and
XRProjectionLayer instead of the WebGL-only XRWebGLLayer, while
preserving the existing WebGL XR path as the default fallback.
New files in src/core/xr-webgpu/:
- xrGpuTypes.ts: TypeScript declarations for XRGPUBinding spec types
- xrGpuSessionSetup.ts: GPUDevice access and session init helpers
- xrGpuTextureProvider.ts: Per-frame GPUTexture swap via hwTex.set()
- xrGpuLayerWrapper.ts: WebXRLayerWrapper subclass for projection layers
- xrGpuRenderTarget.ts: WebXRRenderTarget using XRGPUBinding
- xrGpuEntryPoint.ts: Public API for availability check and creation
Modified xrEntryHandler.ts to conditionally route through WebGPU or
WebGL XR entry based on XRGPUBinding availability.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
