Function fetch_response_mutation 

pub fn fetch_response_mutation(
    fence: FetchFence,
    response: FetchResponseFacts,
    content: FetchContent,
) -> FetchResponseMutation

Fence a Fetch response against the live role, leader, and epoch, then select exactly one mutation path.

A discovery response carries no mutation: the node attaches to the advertised leader and fetches again under the leader fence, which admits content only from that leader itself. So no response is ever applied on the strength of who the responder is assumed to be.