Version, content hash and status of the field registry in use

Editor and above, or an API key with the `condition` scope (source `api`, ADR-016, ADR-017). Merge semantics: only the named fields change, `null` clears a field. Each value has a `measured_at` (its own or the request's), which must not lie in the future or before the manufacturing date. Fields that are not dynamic fields of the unit are refused (`unknown_field`); so is the whole update when a value is invalid or the merged condition data fails a cross-field check. A value older than the stored one, or equal to it, is skipped and listed. Accepted values are stored as approved measurements and recorded as a `condition_update` lifecycle event; a published passport is re-published as the organisation's `condition_republish` setting says (immediately in this request, or by the daily job).