Module sources 

Source implementations.

Each source is a concrete Source trait impl. Per docs/SOURCES.md §1:

• Tier 1 (Open Access, always compiled in): Crossref / Unpaywall / arXiv.
• Tier 2 (metadata enrichment, Phase 4, behind the metadata Cargo feature): OpenAlex / Semantic Scholar / DOAJ.
• Tier 3 (TDM, Phase 5, behind per-publisher Cargo features): Springer Nature OA / APS Harvest / Elsevier ScienceDirect.

Modules

arxiv

arXiv source — arXiv id → PDF + Atom-feed metadata.

crossref

Crossref source — DOI metadata + OA URL discovery via link[] array.

unpaywall

Unpaywall source — OA URL discovery + license metadata for a given DOI.