Bruno Stefanini, a Swiss real estate magnate who died in 2018, spent his life collecting huge numbers of buildings, fine art and historic memorabilia, everything from castles to paintings to the toothbrush Napoleon is said to have used at Waterloo. The collection underwritten by his fortune became massive — more than 100,000 pieces — and it included 6,000 oil paintings, many of them by important Swiss artists like Augusto Giacometti and Ferdinand Hodler. “The sheer numbers are slightly overwhelming,” said Carolin Lange, head of provenance research at a foundation Stefanini created in 1980 in an effort to shepherd it all....