Drives the real gateway against a running Objecten + Objecttypen pair: two
writes for the same id leave exactly one object carrying the second write's
status, and nothing outside the public-safe schema. Runs under verify-acl,
inside the compose network — which it must, because Objecttypen echoes the
request Host into the objecttype `url` and Objecten only accepts the one
matching its configured api_root. ADR-0028 records that constraint.
