Move the flat z-* / trade-doc pages into section folders so the sidebar
becomes a set of collapsible groups instead of one long flat list:

- Trade documents -> app/docs/<slug>/ (leaf slugs unchanged).
- z-* narrative pages -> governance/, architecture/, use-cases/,
  business-cases/, evidence/, implementation/, register/. Each section's
  overview is the folder's own page.mdx (so /use-cases/ is the overview).
- Top-level app/_meta.js lists the sections; each folder has its own
  _meta.js. Separators + z- prefix dropped.
- All internal page links rewritten to the new hierarchical paths
  (scripts/restructure.mjs is the single source of truth for the map).
- public/_redirects 301s every old flat URL to its new path (GitLab Pages).
- Fix a latent React root lifecycle bug in JsonSchemaViewerShadow: use a
  fresh container div per root and defer teardown, so dev StrictMode /
  fast-refresh no longer throws the createRoot/unmount console errors on
  the /docs/ pages.
- Update CLAUDE.md for the new page-location convention.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>