Function check_trifecta 

pub fn check_trifecta<I>(tags: I, allow_trifecta: bool) -> TrifectaVerdict
where
    I: IntoIterator<Item = TrifectaTag>,

Pure Rule-of-Two check. Folds the tags of a granted tool set (OR across legs) and judges the accumulated budget:

• fewer than three legs → TrifectaVerdict::Ok (any two is fine);
• all three legs → TrifectaVerdict::RefusedTrifecta, unless allow_trifecta downgrades it to TrifectaVerdict::AllowedWithWarning.

Structural only — it never inspects tool content and never asks the model to judge; it is a budget on co-located capability. Call it over the tags of the already-narrowed grant and before the grant is handed out: refusing once a child already holds the capability is too late.