crossref per-source checkers swallow all exceptions to [] — a broken adapter looks like a clean record #71

Each crossref source checker catches broad exceptions and returns an empty match list (e.g. crossref.py:330-331 and siblings), so an adapter that's crashing reads as 'entity not found in this database.' For an investigation tool this is the same class of failure as a silent search empty: absence of evidence gets manufactured. Proposal: per-source status in the report — checked | matched | ERRORED(reason) | skipped(offline) — and a nonzero summary line when any source errored. The _RateLimited(BaseException) trick (crossref.py:87-92) shows the codebase already knows blanket excepts are a hazard.