Invisible proof-of-work challenge (the default).
The browser must find a nonce such that SHA-256(salt <> nonce) starts
with difficulty zero bits. Legitimate users wait about a second; bots
pay that CPU cost on every submission. The server verifies with one hash.
Requires the wiztchta JS (see the README) and a secure context
(HTTPS or localhost), since it uses crypto.subtle.
Options
:difficulty- leading zero bits required. Each +1 doubles the average work. Defaults to the:pow_difficultyconfig, or16.