OpenAPI $ref resolver

Inline every local `$ref` in an OpenAPI 3.x or Swagger 2.x document so downstream tools see a self-contained spec. Resolves `#/components/...` (OpenAPI) and `#/definitions/...` (Swagger) JSON-pointer refs anywhere in the document, including inside components themselves. Per-branch cycle detection: if A→B→A, the second A is left as a `$ref` and recorded under `circular`. External refs (http://, file://, ./other.yaml) are never fetched - they're reported under `external` and left as-is.