Adds example_records(), a bundled harvest of 138 real journal articles on graphene supercapacitors over the standard RECORD_COLUMNS schema, the same corpus the R twin ships. The records come from OpenAlex under CC0, since the Elsevier terms do not permit redistributing a Scopus harvest.
Every guide, reference page and the app's demo mode now run on that corpus instead of a fabricated frame, with the live API call shown alongside.
Fixes extract_dois(), which emitted the literal string <NA> for a record without a DOI because its guard caught only float NaN while the corpus uses a nullable string column.
Full notes in CHANGELOG.md.