RFD 1009 details: the renderer fallback chain

This replaces the reference this RFD once made to docs/THREEJS_WEBGPU_WEBXR_MIGRATION.md. That guide, and its companion THREEJS_QUICK_START.md, described a getRendererInfo() method, a setupPostProcessing() call with SSAO, Bloom, and FXAA toggles, and a createPositionalAudio() spatial-audio helper. None of the three exist in sceneManager.js. EffectComposer appears only in screenshotManager.js, for a screenshot render pass, not a live post-processing pipeline. Both guides drifted from a build that never shipped, or shipped then shrank; deleted, not rewritten.

The renderer fallback chain

SceneManager.initialize() calls createViewportRenderer(), which returns a WebGPU renderer, a WebGPU-support flag, and a type string, when the browser and GPU support it. this.rendererType takes that value: 'webgpu', 'webgl', or 'software', falling back in that order when a step fails. initialize() returns rendererType alongside scene, camera, renderer, and controls, so a caller can read which tier is active.

What the old migration doc got right

The VR camera offset and the AR pass-through transparency it described are real, current behavior. RFD 1108 gives the exact wrapper math and the reference-space priority, more precisely than the old guide did.