Rux runtime shell, milestone M3.
Opens a native window (winit), manages the GPU via vello's RenderContext,
loads a .rux document each frame's tree from rux-runtime, and paints it
(rux-paint). A notify file watcher wakes the event loop through an
EventLoopProxy on every save, so edits to the .rux file repaint live,
the hot-reload path from docs/04-architecture.md.