Function ensure_engine 

pub async fn ensure_engine(
    dest: &Path,
    on_progress: impl Fn(EngineFetchProgress) + Send + Sync,
) -> Result<PathBuf, String>

Ensure a managed engine binary exists under dest (the data-dir engines/), fetching the release for this host’s OS/arch if absent. Returns the path to the engine binary; reports progress through on_progress.