Function dispatch_sort_key 

pub fn dispatch_sort_key(
    candidate: &PrefetchCandidate,
) -> (u32, u8, Reverse<Option<u64>>)

Sort key for dispatch order, lowest first (#617).

Lexicographic, deliberately, rather than a weighted score: a weighted sum needs coefficients, and nothing can calibrate them until #618 makes “arrived before it was demanded” measurable. Every element here is an ordering, not a magnitude.

1. Urgency bucket. Prefetch races the build, so a high-value artifact needed at minute eight loses to a medium-value one needed at second five. Candidates with no demand index sort last.
2. Confidence. Within one wave, prefer the source most likely to be right.
3. Value, descending. Expensive rebuilds first, so the limited slots buy the most avoided work. Unknown cost sorts after known cost rather than being scored as zero.

Callers must apply this as a STABLE sort: equal keys keep source order, which is the planner’s confidence-merge order.