[v11] Verify RenderTexture: render-target save/restore on WebGPU

What it does

src/core/Portal/RenderTexture/RenderTexture.tsx:195-219 saves the current render target, draws the portal scene into its own FBO, then restores:

oldRenderTarget = state.renderer.getRenderTarget()
state.renderer.setRenderTarget(writeFbo as THREE.WebGLRenderTarget)
state.renderer.render(portalScene, portalCamera)
state.renderer.setRenderTarget(oldRenderTarget as THREE.WebGLRenderTarget | null)

What to check

• The getRenderTarget() / setRenderTarget() round-trip. On the WebGPU Renderer the current target is this._renderTarget; confirm getRenderTarget() returns something setRenderTarget() accepts back, including when it is null (the default framebuffer).
• The as THREE.WebGLRenderTarget casts on lines 217 and 219 are false under WebGPU — the object is a RenderTarget. They compile only because the cast silences the checker. They should route through #drei-platform or be typed as the union, so the type says what is actually there. This is a live example of a cast hiding a renderer-split problem.
• Double buffering: the component keeps writeFbo / readFbo. Whether a WebGPU frame boundary changes the swap timing is worth checking.

Shared context (identical across all seven)

These components are classified agnostic — they live in core/, so they are assumed to work on both renderers. That assumption held for 104 of the 106 agnostic components when swept: 73 never touch the renderer at all, and 31 touch only members both renderers have. These seven are different: they drive their own render calls, and that is where semantics diverge even when the API surface matches. See #2818.

setRenderTarget(renderTarget, activeCubeFace = 0, activeMipmapLevel = 0) has an identical signature on both (Renderer.js:2578 vs WebGLRenderer.js:2890), so that part is fine.

render() is where they differ — three/src/renderers/common/Renderer.js:1362:

render( scene, camera ) {
  if ( this._initialized === false ) {
    throw new Error( 'THREE.Renderer: .render() called before the backend is initialized. Use "await renderer.init();" before rendering.' );
  }
  this._renderScene( scene, camera );
}

WebGL has no such gate. A render() that happens before the backend is ready throws on WebGPU and silently works on WebGL.

This is the class of bug that produced #2809: Preload calls gl.compile(), which on WebGPU is a getter aliasing compileAsync — same name, returns an unawaited promise, so preloading never happens. Grep cannot find these; they need reading.

Acceptance

Read the component against the WebGPU Renderer implementation and either confirm it is correct or fix it and say what was wrong. A story exercising the render path on WebGPU is the evidence — note Setup sets frameloop: 'never' under test, so useFrame does not run there and a green story proves nothing about this.