HARRISBURG — How much are state and national groups spending to flood the airwaves, fill mailboxes, and buy up digital ads ahead of the Pennsylvania Supreme Court retention elections? That’s a harder question to answer than you might think, thanks to a mix of lagging reporting, weakly enforced rules, and a campaign finance system that requires little transparency. Many groups are trying to sway voters through independent expenditures — money spent without coordinating with the candidates’ campaigns. These expenditures are often made by groups that can’t legally give to candidates, including dark money nonprofits that don’t have to disclose their...