Function encryption_audience 

pub fn encryption_audience(scope: &Scope) -> String

The encryption audience a scope maps to — the set of principals whose key a payload under this scope is encrypted to. Personal → the user’s own key; Shared{org} → the org key. The B4-pinned rule (“scopes are encryption audiences”) uses this: a single ciphertext must have a single audience, so a whole-chain checkpoint mixing scopes cannot be one ciphertext.