New tools:
- pubmed_lookup_citation: resolve partial citations to PMIDs via ECitMatch
- pubmed_convert_ids: convert between DOI, PMID, PMCID via PMC ID Converter

Service changes:
- Reusable withRetry() method, makeExternalRequest() for non-eutils APIs
- HTTP error classification: 4xx → InvalidRequest, 5xx → ServiceUnavailable