🔗 docs: link the live site, now that Pages is confirmed

Pages serves docs/ from main, so the page rebuilt in #6 is live at
avalon-vanguard.github.io/cereale. The README pointed at the local file
because the URL could not be verified from here; it now links the site and
keeps the local instructions as the fallback. package.json homepage moves
there too — npm renders it as the package's headline link, and a live
playground is a better landing spot than an anchor inside the README.

Adds canonical and Open Graph tags. No og:image: a preview card with a
broken image is worse than one without, and there is no artwork yet.

check-docs.mjs flagged the canonical link as a remote subresource, which it
is not — the browser never fetches it. Rather than exempt the URL, the
check now looks at rel and only flags the relations that actually fetch or
connect. Verified it still catches a CDN stylesheet, a preconnect and a
script src; a check that cannot tell a declaration from a request is one
that gets switched off the first time it is wrong.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SAcqrz3FcadkYr3xG32CjK

README.md

@@ -25,9 +25,10 @@ const user = fromJsonSync(User, body);   // a real User
 user.greet();                            // your methods are still there
 ```
 
-`docs/index.html` is a self-contained page with an interactive playground that runs this
-library in the browser. Build its assets with `npm run build:docs` and open the file — it
-loads nothing from the network.
+**[avalon-vanguard.github.io/cereale](https://avalon-vanguard.github.io/cereale/)** — an
+interactive playground that runs this library in your browser, the full decorator reference,
+and the toolchain matrix. The page is self-contained and loads nothing from the network; it is
+served from `docs/` on `main`, and `npm run build:docs` rebuilds its assets to open locally.
 
 ## Where it fits