Function transient_builder 

pub fn transient_builder(
    client: TransientClient,
    registry: CommandRegistryHandle,
    plugin: String,
) -> impl Fn(&TransientContext) -> TransientBuildFuture + Send + Sync + 'static

The registry builder for a transient plugin: projects the open context synchronously, then awaits the guest’s build on the plugin’s own actor task and converts the result.

The synchronous prelude matters — it is what lets the returned future be 'static, so nothing borrows the editor across the await.