nx_pow_challenge.nx

nx_pow_challenge.nx -- proof-of-work client puzzle for the access wall's L7 DDoS shedding (hashcash; Cloudflare "under attack"). Under high load/anomaly the server issues a fresh CHALLENGE + a difficulty D; the client must find a SOLUTION such that sha256(challenge || solution) has >= D leading zero bits. The server VERIFIES in O(1) (one sha256) but the client pays O(2^D) expected work -- the asymmetry that makes a flood expensive for the attacker and a one-time cost for a legit client. Reuses the KAT-verified nx_sha256 (no rolled crypto). license_tier: ORIGINAL