Checks the `MUST`-contents of the configuration file specification, see <https://codecheck.org.uk/spec/config/latest/>.
Details
The structural checks are the rules of the specification version the file declares, run by [validate_codecheck_yml_rules()] - this function does not implement them a second time. It stops at the first failure, as it always has, whereas [validate_codecheck_yml_rules()] reports every rule.
Which rules are enforced here is deliberately the set this function has always enforced, see `VALIDATE_YML_RULES`: it gates the register rendering, so widening it rejects certificates that are recorded and published today. Use [validate_codecheck_yml_rules()] for the full picture of a file.