find_gaps defaulted its probe topics to paper titles, so a title — which
retrieves mostly its own single bibkey — fell below synthesis_gap_min_coverage
and flagged almost every paper as a 'gap'. Coverage-map gaps now run only for
explicitly-requested topics; the CLI gains a repeatable --topic option. The
code-vs-corpus gap path (precise) is unchanged and still default.
Tests: the two cases that relied on default-title coverage gaps now pass explicit
topics; added a regression that no coverage gaps appear without --topic.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>