Threenix Background Node

Add one scene-level TSL background through scene.backgroundNode using the bundled assets/BackgroundNode.tsx reference.

Workflow

1. Read the target repository's local instructions and inspect the existing Canvas, scene composition, and package versions.
2. Confirm the target uses @react-three/fiber/webgpu with useLocalNodes. If it has a WebGL Canvas, stop and explain that migrating renderers is out of scope.
3. Copy assets/BackgroundNode.tsx into the target's scene component location. Keep the component local to the Canvas and render it once as a child of that Canvas.
4. Replace only the example TSL graph inside createBackgroundNode with the requested visual. Keep the graph in three/tsl, return it as backgroundNode, and preserve <primitive attach="backgroundNode" object={backgroundNode} />.
5. Reuse the scene's existing textures, uniforms, and shared nodes where they fit. Use useLocalNodes for component-local node graphs; do not add a full-screen mesh, a second Canvas, raw GLSL, or a postprocessing pass just to draw the background.
6. Add only missing dependencies required by the existing R3F WebGPU setup. Do not add controls, UI, fallback renderers, or a texture pipeline unless the requested look requires one.
7. Run the target repository's narrowest type-check or build covering the scene and report any unresolved compatibility issue.