The stored steps (what a step ends on in the module's `steps` state) of a fetch (fetch.json), a candidate's build
(build-candidate.json, no publish) and a trunk build (build-trunk.json, publish), as buildstep writes them, with ids, times,
the workspace, the commit, digests and the registry's address put to fixed words (STEPID, WORKSPACE, COMMIT, REGISTRY, one
sha256 digest). Written by capture_test.go (MESH_UPDATE_CAPTURED=1) from the unit tests' fake runner on 2026-10-22, and
compared by bus_integration_test.go with the steps a real bus holds. mesh-delivery-builder's tests copy these files.
