The GPU compositor goes live: engine frames present on screen

Since 0.1.132 the GPU compositor has blended layers on the device byte-for-byte with software and proven it against an offscreen texture, but it was dark at runtime - the shipping path still encoded every frame to PNG and handed it to the webview. This wires the compositor into the live shell: opt in with an environment flag and the browser opens a real window with a wgpu swapchain over it, and every frame the engine paints is presented straight to the window texture with no PNG, no base64, and no webview image decode. Confirmed on screen through the app - a page rendered by the from-scratch engine presenting directly through the swapchain. The GPU aspect moves 4 to 6; it stays modest because rasterisation, WebGL, and video are still on the software path, and the default shipping path is unchanged. The overall holds at 57.