hexa can now call the audio API with types, not guesswork.

- clients/openapi.json — checked-in OpenAPI 3.1 snapshot (servers pinned to the
  public https://api.nech.pl/audio/v1), covering all 13 routes. A snapshot-drift
  guard test asserts it stays in sync with the live app (add a route → refresh or CI fails).
- clients/douanier.ts — a typed, ZERO-dependency client (global fetch/FormData/Blob;
  works in Node 18+, Vercel Functions, Edge). One method per engine (emotion,
  features, samples, grade, onsets, waveform, analyze, loudness, spectrum, naming),
  typed results, X-Douanier-Cache surfaced as result._cache, DouanierError on non-2xx.
  Typechecks clean under tsc --strict.
- clients/README.md — base URL + bearer, a Vercel Function example, the endpoint/
  scope table, curl, and the openapi-generator one-liner for full codegen.

60/60 tests (added the snapshot guard). The API is now self-describing
(/audio/v1/docs + openapi.json) and has a drop-in client.