Function register_probe 

fn register_probe<'a, T, F>(
    builder: PluginBuilder<'a>,
    name: &str,
    data: T,
    probe: F,
) -> PluginBuilder<'a>
where
    T: Send + Sync + 'static,
    F: Fn(&T) -> bool + Send + Sync + 'static,

Register a () -> i64 boolean host function whose answer is probe(data). data is captured host-side from the grant/context — never supplied by the module — so the probe is scope-checked by construction.