The SHA-256 every integrity check compares: lowercase hex, 64 characters.

Builds the `ex-info` every SDK failure is raised as.

The `:type` of its `ex-data` names the failure (`:ankusa.sdk/ClaimNotFoundError`)
and the rest of the data is that type's fixed key set: every key is present,
`nil` when it does not apply, so callers can read any of them without a
`contains?` guard. See `ankusa.sdk.errors`.

Reading request headers the way every receiver-side helper does.

The one place the SDK issues requests.

Conventions every caller relies on, because the conformance vectors count
requests and bound latency:

* One request per call. There is no retry.
* A redirect is never followed: a `3xx` is classified as `unavailable`.
* A non-2xx status is a value each client maps to its own error, never an
  exception.
* Bodies stay bytes; the caller decides whether they are claim bytes, JSON,
  or Prometheus text.