Function budget 

pub fn budget<'a>(
    ranked: &[(f64, &'a Challenge)],
    done: &HashSet<&str>,
) -> Vec<&'a Challenge>

The Stop’s two decisions over a ranking of the whole store (best first, as rank returns it; the top is the store’s best match, done or not), with done the lessons this session already asked, or delivered early and acted on (R3b, internal evaluation “phase-r”, not published):

• whether to block (the cost: one more round): only when a lesson not done scores at least BLOCK × the top. A context whose best lessons were delivered early and acted on does not block.
• what to ask once blocking (cheap): every lesson not done scoring at least LOW × the top, scoped lessons first (rank already dropped those out of scope), at most GENERIC_CAP unscoped, CAP in all, ordered by score × the session’s usefulness draw (R1, explored).