Function host_stub 

fn host_stub(
    _plugin: &mut CurrentPlugin,
    _inputs: &[Val],
    outputs: &mut [Val],
    _user_data: UserData<()>,
) -> Result<(), Error>

Placeholder body for host functions whose real implementation lands in a later slice (foreground-window and the export sinks). Returns 0. Its mere presence is still gated by the capability, so registering it doesn’t widen the module’s reach.