v6.0.0

• feat!: report database failures and fix broken adapters

BREAKING CHANGE: findVerifiedPapers with no results now returns JSON (count, message, sourcesQueried, errors) instead of a plain-text sentence, and is flagged isError when every database queried failed. Unverified verifyCitation and batchVerifyCitations results now include errors.

Database failures are no longer swallowed. Before, a rate limit or outage came back as "No verified papers found", which reads as "no such paper". Responses now name each failed database and say why.

Each database is now asked for 3× limit results, then trimmed back to limit after dropping records without a DOI, so DOI-less top hits no longer empty a source.

Adapters fixed after live testing (single-source, limit 2):

• HAL: asked for no fields, so no record had a DOI. Now requests fields and only DOI-bearing records.
• ERIC: has no DOI field. DOIs are now read from doi.org links in url, and the search is limited to such records.
• zbMATH: the old endpoint returned HTTP 400. Moved to the zbMATH Open v1 search API.
• INSPIRE-HEP: sorted by most recent, which surfaced preprints without DOIs. Now sorted by relevance.
• Semantic Scholar: the shared anonymous quota answers HTTP 429. Set SEMANTIC_SCHOLAR_API_KEY for a dedicated quota; a 429 is reported as RATE_LIMITED with that hint.
• DBLP: dblp.org now serves a bot-check page instead of JSON. This is reported as an explanatory error; the server does not try to get around it.