Feat/mic 73 openapi model

[matthewmr-eqty]

Need

Customers read guardian's three APIs on the current docs site and hold links to 146 of its pages. The viewer needs one model of each service before it can render any page.

Problem

Two of the three files are Swagger 2.0. Circular $refs cannot be stored as content. Customer links follow starlight-openapi's slug rule, which the design's own wording got wrong.

Change

A pure module with no Astro imports. Slugs copy starlight-openapi's rule exactly, and schemas become a finite tree that marks true cycles only. Samples and the Markdown twin come from the model, so a page and its twin cannot disagree. Nothing imports the module yet; the loader and pages follow in the next two PRs. swagger2openapi plus swagger-parser was rejected.

Evidence

All 146 live URLs reproduced from the pinned files. 28 new tests, including zero false recursion markers across the three real specs. Lint, format check, build and the full test suite (131 docs tests) pass on top of #157.

Not covered

No page renders yet. The model has not run inside Astro.

Flow

_api.json / _api.yaml
      │
   ingest ── Swagger 2.0 → OpenAPI 3, dereference
      │          └─ bad file → OpenApiError(file, line, fix)
      ▼
   buildModel ── slugs, tags, finite schema tree
      │
   ApiModel ──┬─ samplesFor → cURL, fetch
              └─ markdownFor → twin body

Review focus

Read src/openapi/ in this order: ingest.ts, model.ts, schema-tree.ts, then samples.ts and markdown.ts. The 13 files under tests/fixtures/openapi/ are byte copies of guardian at e87db18be, kept out of Prettier on purpose. pnpm-lock.yaml can be skipped.