A Hayagriva exporter exists, but there is no corresponding importer. Implement one in org.jabref.logic.importer.fileformat.

1. Create the importer in package org.jabref.logic.importer.fileformat
2. Use basic.yml from the upstream test suite as the primary test file
3. Add a round-trip test (export → import → compare) once Issue Hayagriva exporter writes DOI as plain field instead of serial-number #15713 is fixed, covering all serial-number subtypes (DOI, ISBN, ISSN, …)

Follow-up to #15713