Check that ISBNs in cross-references are valid

When an ontology uses ISBNs in cross-references (typically, to indicate the source of a definition or a synonym), the ODK could provide a test to ensure that all ISBNs mentioned in cross-references (identified with a `ISBN:` prefix) are actually valid.

Start by locating the ODK test or validation entry point that processes cross-references, then inspect how existing validation checks are structured. Confirm how `ISBN:` references are represented and define completion as reporting invalid ISBNs while accepting valid ones.

Written by the indexing model from the issue text.