Client-side cap on the PoW difficulty it will attempt to solve (H3). An
unauthenticated HelloRetryRequest carries difficulty verbatim; without a
cap an injected difficulty = 255 would make the client spin ~2^255 hashes
forever. 24 sits strictly above every difficulty an honest server issues
(load-tier max 16, ReputationTracker::MAX_DIFFICULTY 20, the frozen
difficulty: 20 wire vector) yet is a sub-second solve (~2^24 hashes).
Hard iteration bound for PoWChallenge::solve (H3) — 2^32, ~2^8 expected
attempts of headroom over the worst in-cap difficulty (2^24), so a
legitimate solve effectively never spuriously fails while an infeasible one
still terminates.