Add an integrity footer to chat.html, zebra-audio.html, and how-it-works.html
showing the build date (2026-05-28) and the page's own MD5 + SHA-256. A file
can't hold its own hash, so web/stamp.js (make stamp) computes the hashes with
the two hash fields zeroed, then writes the real values back — self-consistent
and idempotent. To verify a served page: blank the two fields and re-hash;
confirmed it reproduces the stamped value with plain sha256sum.
