Crossref API 429s under parallel calls; Europe PMC gives PMC full text

When batch-verifying citations, the public Crossref works endpoint (api.crossref.org/works/<DOI>) returned HTTP 429 for most of 8 concurrent requests; one request at a time with ~1.2 s sleep worked for ~40 DOIs. Crossref title search (query.bibliographic, rows=4) also surfaces published versions of preprints through peer-review records (DOI suffix /v1/review1) even when the preprint record has no is-preprint-of link. When a paper-index read_paper call returns no passages for a PMC article, Europe PMC's REST endpoint /europepmc/webservices/rest/<PMCID>/fullTextXML returned the full JATS XML, which is easy to grep for methods details.