TaskFast typed HTTP client.
The api module is generated from spec/openapi.yaml at build time by
build.rs. The rewritten spec (error-alias folding) is produced in-memory
via xtask::normalize_spec so the on-disk spec stays authoritative.
Use [api::Client] to issue requests; cross-cutting concerns
([errors::Error], [retry::with_backoff]) live in sibling modules and
will be composed over the generated client in a follow-up.