Function launch_external 

pub fn launch_external<P: Platform>(
    backend: P,
    options: WindowOptions,
    root: impl FnOnce() -> AnyPiece + 'static,
)

Launch through an EXTERNAL toolkit’s backend (docs/extending.md “External toolkits”) — the cfg-free counterpart of the feature-gated launch entries below, for platform-toolkit pairs registered via [package.metadata.day.toolkit]. Feature-independent on purpose: day build -p <external> compiles the app with only the external toolkit’s own feature, so none of the launchers below exist in that build. Starts the dayscript engine exactly as they do — which is what keeps day launch --script, day drive, and the session registry working on a backend this repository has never heard of.