1. Create a new library
2. Add an entry with a DOI, e.g. 10.1109/EDOC.2018.00030
3. Export as Hayagriva (.yml)
4. Open the exported file

Expected at step 4:

1. Identifier fields (DOI, ISBN, ISSN, …) are nested under serial-number:

serial-number:
  doi: 10.1109/EDOC.2018.00030

Current behavior:

doi: "10.1109/EDOC.2018.00030"

Per the [Hayagriva file format spec](https://github.com/typst/hayagriva/blob/main/docs/file-format.md#serial-number), all such identifiers must be nested under serial-number. Fix should include unit tests covering each identifier type (DOI, ISBN, ISSN).