ShaderToy feedback export notes

- This is intentionally not a fake single-pass demo.
- If the original effect uses Buffer A/B/C/D or previous-frame state, plan render targets first.
- For raw WebGL: expect ping-pong framebuffers.
- For Three.js: expect render targets and pass ordering.
- For R3F: keep feedback state outside casual component state and make the pass graph explicit.
