slate: lifecycle-couple the launcher to slate-shell (single-program run)

Slate couples its launcher to slate-shell via die-with-parent and cooperative polling, so closing the shell exits Slate and killing Slate closes the shell. The launcher keeps serving the PWA and WebSocket during this async sleep, while open-renderer! returns the process handle for lifecycle coupling. This single-program run persists through port-busy adoption by detaching the window from the new launcher.