visible_to

def visible_to(envelope: Envelope, participant_id: str) -> bool:
    """Pure delivery / view-filtering predicate.

    Sender always sees their own envelope; broadcasts (``audience=None``)
    are visible to all session participants; subset addressing is
    visible only to listed peers.
    """
    if envelope.sender_id == participant_id:
        return True
    if envelope.audience is None:
        return True
    return participant_id in envelope.audience