Development record

The own-artefact refusal is extension- and parse-shaped: it keys on a .json extension and a successful top-level unmarshal, so a prior manifest or bundle committed as .yaml or .toml, or as .json with a BOM, wrapped in another object, wrapped in an array, or carrying a duplicate _type key, is admitted whole while the new manifest asserts the exclusion. Make the check content-signed: refuse any admitted file whose bytes contain either artefact _type tag string before any parse, regardless of extension, keeping the parse as a secondary check.