🔗 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

package.json

@@ -68,7 +68,7 @@
   "bugs": {
     "url": "https://github.com/avalon-vanguard/cereale/issues"
   },
-  "homepage": "https://github.com/avalon-vanguard/cereale#readme",
+  "homepage": "https://avalon-vanguard.github.io/cereale/",
   "devDependencies": {
     "@babel/standalone": "^8.0.4",
     "@eslint/js": "^10.0.1",