feat: strengthen documentation loading and link validation

[LunaStev]

Documentation navigation repeatedly fetched the same catalogues, and translated headings containing punctuation could produce a section index that did not match the rendered article. Missing documents also showed the same retry state as service failures.

Public catalogue requests now share in-flight work and reuse successful results for one minute within the documentation session. Errors are not cached, and explicit retries refresh the catalogue. Article and catalogue requests run concurrently; English fallback still requires a translated-document 404. Missing pages offer a route back to the catalogue, while other failures remain retryable.

The section index now comes from the sanitized rendered article. Fragment links restore their target after the article and mobile index have rendered, including refresh and translated headings. CI validates document paths and actual Markdown heading IDs across all 486 official documents, including reference links, redirects, duplicate paths, supported locales, and English fallback. Markdown source-file links are rejected because the platform serves documentation routes.

Validation:

• Frontend documentation regression tests, rendered-link checks, and production build.
• Catalogue request coalescing, expiry, explicit refresh, failure recovery, locale isolation, and 404-only fallback tests.
• Docker Compose rebuild and healthy local services.
• Browser checks for a Spanish punctuation heading, direct fragment entry and refresh on desktop/mobile, missing-page recovery, and English fallback.
• Existing documentation link/example checks and whitespace checks.