docs(blog): signed exact-ref digits weekly demo

[enricopiovesan]

Summary

• New weekly blog signed-exact-ref-digits: signed digits-mlp-1.0.0 exact-ref package, same bytes on Browser + Node (ExactModelBrowserHost), tamper → digest_mismatch.
• Screenshots converted to webp under /assets/img/blog/signed-exact-ref-digits-2026-10-02/.
• Blog index featured prepend + light llms.txt update.
• Public story stays blog-first. Links the private weekly-demos tree for people who already have access. No public-clone CTA (.github#30).

Honesty

• Test-only signing key; production signing is traverse#1567.
• Not Hugging Face. Not catalog model.execute.
• Node column is the published web embedder under Node, not a native Rust CLI.
• Swift / Kotlin / .NET exact-ref execute is not claimed on the published v0.14.0 pins this demo uses.

Test plan

• cla
• governance baseline
• node --test
• playwright e2e