Turn each OpenAPI page into sidebar entries that follow edits in dev

[matthewmr-eqty]

Need

A service's API reference should appear in the sidebar and in every page-listing feature (twins, search, versions) like any other page, and stay current while an author edits in astro dev.

Problem

The glob loader rewrites entries on its own schedule, one file watcher serves all 27 content collections, and the nav derives placement from the entry id, which splits operation ids into folders and title-cases tag names.

Change

docsLoader fans each openapi: page out into a model entry and one entry per operation, re-expanded on spec or page edits, serialised per page. Operations inherit draft, hidden and noIndex. Generated entries carry their sidebar placement; tags arrive as virtual groups; method badges are text-only. A spec outside the content directory, or a generated URL that collides with an authored page, fails the build. A route-side helper was rejected: it would bypass every page-listing feature.

Evidence

155 docs tests pass, 24 of them new. In a live astro dev on this branch: adding and removing a path updated the sidebar without a restart, deleting the file removed its rows and logged the missing path, restoring it brought them back, prose edits and restarts kept the rows, and saving _group.yaml no longer logs a schema error. Lint, format check and build pass.

Not covered

No operation page renders yet; routes and components are the next PR. Node warns about more than 50 watcher listeners on dev start.

Flow

edit _api.json or api.mdx
        │
  shared watcher ──▶ glob loader (drops _ files)
        │
  docsLoader listener ──▶ expand(page), one run at a time
        │
  store: page/~api + page/operations/<slug>
        │
  docsNav ──▶ page group ▸ tag groups ▸ GET rows

Review focus

classify in src/nav.ts changes for every site: pages without navIndex or navPlacement take the old path unchanged, and the existing nav tests are only appended to. Read in this order: openapi/fan-out.ts, openapi/expand.ts, loaders.ts, then nav.ts and runtime/lib/nav-data.ts.