Function apply 

pub fn apply(
    plan: &Plan,
    challenge: Challenge,
    issuance: Issuance,
    uid: &str,
) -> Result<Vec<Step>, (Vec<Step>, String)>

Performs the privileged half, in the order that survives a failure at any point.

Marker before the hook runs, hook installed before the first projection: the hook reads the marker to decide whether the node needs the pair at all, so running it first would skip the projection and leave a marker naming two files that are not there. Running the hook is the first projection — proving the renewal path today, rather than at a renewal two months from now when nobody is watching.

Errors

Returns the steps completed so far alongside the failure, so the caller can print what was done as well as what stopped.