Adapt reflection and screenshot components to WebGPURenderer

[vincentfretin]

Makes the reflection and screenshot components work with WebGPURenderer (three.webgpu.js build, see #5749 for context).

Changes

• Cube render target: WebGLCubeRenderTarget is not exported by the three.webgpu.js build, which uses CubeRenderTarget instead. Both components now select the class at runtime with the same trick a-scene uses to select the renderer implementation, so webpack can't statically resolve the export. This also removes the WebGLCubeRenderTarget webpack warning when building with WEBGPU=true. THREE.CubeCamera works unchanged with either target.
• Equirectangular projection: WebGPURenderer does not support RawShaderMaterial/ShaderMaterial (not in its node library). When THREE.TSL is available, the cubemap-to-equirectangular projection is built as a MeshBasicNodeMaterial with TSL nodes implementing the same longitude/latitude math as the GLSL shaders; the cube texture is swapped per capture through the texture node's .value.
• Pixel readback: WebGPURenderer only supports asynchronous read back. renderCapture uses readRenderTargetPixelsAsync when readRenderTargetPixels is not available. getCanvas() still returns the canvas synchronously with WebGLRenderer (no breaking change) and returns a Promise resolving to the canvas with WebGPURenderer (documented in screenshot.md). capture() works on both.
• RGBAFormat: the equirect cube render target used RGBFormat, which is not color-renderable with the WebGL 2 backend of WebGPURenderer (incomplete framebuffer, black capture). Switched to RGBAFormat; the projection outputs alpha 1 and the readback path already assumed 4 bytes per pixel.

Verification

Tested with a scene with distinct sky/box/ground colors on both builds (npm run dev and npm run start:webgpu) driving headless Chrome, probing pixels of the resulting canvas:

• Perspective and equirectangular captures produce identical colors/orientation on WebGLRenderer and on WebGPURenderer (WebGL 2 fallback backend — no WebGPU adapters on Linux Chrome).
• The reflection component sets a live cube texture as scene.environment on both renderers without errors.
• Scene component tests and lint pass.

Note: the actual WebGPU backend readback couldn't be tested on Linux; worth a quick check of equirect orientation in a desktop Chrome with WebGPU support.

ar-hit-test.js uses the same readRenderTargetPixels pattern but is WebXR-only, so it's left untouched until WebXR support lands for WebGPURenderer.

🤖 Generated with Claude Code

[vincentfretin]

Testing help wanted: on this machine I could only verify WebGPURenderer running on its WebGL 2 fallback backend (Chrome on Linux has no WebGPU adapters). If someone has a machine with WebGPU support (chrome://gpu shows WebGPU: Hardware accelerated), could you test this PR with the actual WebGPU backend?

gh pr checkout 5848
# or without the GitHub CLI:
# git fetch https://github.com/vincentfretin/aframe.git webgpu-reflection-screenshot && git checkout FETCH_HEAD
npm ci
npm run dist
npm run start:webgpu

Then open https://localhost:8080/examples/ (accept the self-signed certificate), pick any example and check in the devtools console that the WebGPU backend is really active:

AFRAME.scenes[0].renderer.backend.isWebGPUBackend // should be true

Things to verify:

• <ctrl> + <alt> + s: perspective screenshot downloads, correct content, not upside down.
• <ctrl> + <alt> + <shift> + s: equirectangular screenshot, correct content and orientation. This exercises the new TSL projection material and the async pixels read back, and is the part I couldn't fully verify: the readback orientation could differ between the WebGPU backend and the WebGL 2 fallback backend.
• Reflection: run document.querySelector('a-scene').setAttribute('reflection', '') in the console, check there is no error and AFRAME.scenes[0].object3D.environment is set.