fix(docs): declare English HTML document language

[zxch3n]

Summary

• Add a Next.js document root that emits lang="en" in server-rendered HTML.
• Add a dependency-free source regression to the existing SEO CI suite.
• Require English document language on every built Markdown/MDX content route in the existing postbuild checks.

Follow-up to #51. The existing English Content-Language metadata does not set the HTML document's language.

Validation

• Source SEO suite: 5 passed.
• Built-output SEO suite: 72 passed, including document-language assertions on all 70 content routes.
• Both new regressions were ablated: removing the source language attribute fails the source check; removing it from generated homepage HTML fails the build check. Restoring the final code/output returns all 77 checks to green.
• Independent scan: all 72 generated HTML files, including error pages, have lang="en".
• Production standalone HTTP checks: homepage, API reference, article, query-string documentation URL and missing-page response all emit English document language; expected four 200s and one 404 verified.
• tsc --noEmit --incremental false: passed.
• Frozen installation with pnpm 9.15.9: passed, with no manifest or lockfile changes.
• The standard production build was killed with exit 137 during compilation. Retrying Next with NODE_OPTIONS=--max-old-space-size=2048 CIRCLE_NODE_TOTAL=2 succeeded, followed by the unchanged postbuild command: sitemap generation, SEO checks and OpenNext Cloudflare packaging all passed. No repository build configuration changed.
• SEO CI passed.
• The local Deno documentation-example suite could not load JSR dependencies. GitHub Deno CI subsequently passed; job logs explicitly confirm 135 tests, 135 passed, 0 failed for this PR's merge-test revision.

The existing Workers Builds check passed and produced a branch preview. All three checks are green for this head; no merged or deployed production outcome is claimed.

Scope

This draft contains only the document root and language regressions. The separate public/llms-full.txt completion item from #51 is not included. No merge, hosting configuration change or manual deployment was performed.

Head: 1b6b83c5b7d5995dc609926f80a82889cccd78aa.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	loro-docs	1b6b83c	Commit Preview URL Branch Preview URL	Oct 02 2026, 07:52 AM