v0.88.2: the template's nav wiring and the arm-index gate

The chassis-nav window: the template's nav wiring and a docs coverage gate, the pre-Topo pass.

Changed

• The showcase's public header nav now renders from site.config.yaml's menus.primary
  (resolved through the engine's extractMenu by the root layout server load) instead of a
  hardcoded array, so an editor's /admin/nav changes reach the rendered site. A lean
  src/theme/site-config.ts module now owns the template's one parseSiteConfig call, and
  cairn.config.ts re-exports siteConfig from it, keeping the full adapter out of the client
  bundle. The footer's nav stays hardcoded on purpose; its list is different content, not a copy
  of the primary menu.
• The tutorial's nav milestone teaches the same shape: parse the site config in a lean module,
  read the menu in a +layout.server.ts, and render from data.nav, instead of importing the
  config module in a client script. The theme tutorial (build-a-theme.md) is now linked from
  the docs front door.
• A new repo gate, check:arm-indexes, fails CI when a published docs page is missing from its
  arm's index (the inverse of check:docs's dead-link direction).

No consumer action is required. The template changes land in newly scaffolded or copied sites;
an existing site keeps its own chrome.