v0.9.1 — arXiv-by-ID verification

Added

• Authoritative arXiv-by-ID verification in bibtex-check. The academic verifier previously matched entries purely by title/author text search against Crossref/DBLP/Semantic Scholar, so brand-new arXiv-only preprints (not yet indexed there) were falsely reported HALLUCINATED/NOT_FOUND with a nonsense best match. FactChecker now extracts the arXiv ID from eprint/archivePrefix or an arxiv.org/abs/<id> URL and fetches the authoritative record from the arXiv export API (new ArxivClient + arxiv_atom_to_record parser), adding it as a scored candidate. Optional FactChecker(..., arxiv=...) parameter — existing callers/tests unaffected. +7 tests.

Fixed (carried from Unreleased)

• S2 arXiv lookups that only tag a preprint with a published venue.
• Silent data loss on non-standard BibTeX entry types.
• Dropped-entry audit trail.

See CHANGELOG.md for full details.