Function check_policy 

pub fn check_policy(
    registry: &PolicyRegistry,
    interner: &Interner,
    subject: &RuntimeValue,
    predicate: Symbol,
    is_capability: bool,
    object: Option<&RuntimeValue>,
    source_text: &str,
) -> Result<(), String>

Run a Check statement against already-resolved subject/object values. Every error string lives here — both engines produce the identical text.