fix(docs): correct SEO routes, metadata, and links

[zxch3n]

Draft completion blocker: public/llms-full.txt is intentionally still at the base revision because the large generated-file upload could not complete. Its old generated links are therefore still stale in this draft. Before merge, install the locked dependencies and run pnpm run extract-llms with Deno available, review the regenerated public/llms-full.txt, and commit it. The complete validated artifact is preserved locally. All other reviewed source and generated-file changes are included.

Summary

• Move API presentation helpers and CSS out of Next's pages directory so the empty helper URLs disappear from routing and sitemaps
• Repair malformed author and Twitter links, including tracked generated content
• Normalize canonicals and advertise Chinese alternates only for 66 source-verified translations; disable the Chinese choice on untranslated pages
• Give the CRDT article a single H1, section H2s, and correct social-image metadata without rewriting technical prose
• Add source regression CI and rendered-HTML postbuild checks, plus a documented translation-manifest update command

This is independent of the knowledge-base content proposal in #39.

Verification

Passed during patch validation:

• pnpm run test:seo: 4 source regression tests; rerun successfully before publication
• pnpm exec tsc --noEmit --incremental false
• CIRCLE_NODE_TOTAL=2 pnpm run build: production build, sitemap generation, rendered checks, and OpenNext Cloudflare packaging
• 72 rendered-HTML tests covering all 70 content pages, metadata, language menus, helper-route removal, article headings, and social image
• Local production HTTP smoke checks: former API helper URLs return 404; API reference and article return 200; query parameters stay out of canonicals; sitemap omits helpers
• git diff --check and clean-baseline patch applicability

Limits and review notes

• The local Deno documentation-example suite could not fetch JSR dependencies. The published PR was subsequently verified by Deno CI run 36666795000: logs confirm 135 tests, 135 passed, 0 failed for head bf4b569638332efa10efd3b54b4cc89f93f2fcc6
• SEO regression CI run 36666794894 also passed for that head
• The first build was killed when Next started eight page-data workers. The successful retry used CIRCLE_NODE_TOTAL=2 to constrain worker count without changing repository configuration
• Optional sharp, old Browserslist data, and read-only global Git configuration warnings were non-fatal
• Build and typecheck results are from the preserved complete-patch validation run on the same base. The published draft omits only public/llms-full.txt as explained above; source regression checks were rerun locally and both GitHub workflows passed
• The translation manifest is pinned to verified source routes; newly translated pages must be added through the documented regeneration workflow
• Visual browser screenshots were not captured

Opened as a draft for review. No merge or deployment has been performed.