Here's the rewritten spec for a single plain HTML file:
Build a cinematic scroll-driven paper plane experience using Three.js (loaded via CDN). The entire experience is a single .html file with embedded CSS and JavaScript. The page is a scroll-controlled animation mapped to a 500vh scroll runway (600vh on mobile below 768px width). No frameworks — just vanilla HTML, CSS, and JS with Three.js. The page has no visible UI chrome — just the 3D scene, text, and effects.