For all its royal history and picturesque location at the confluence of the mighty Mekong and Tonle Sap rivers, Phnom Penh is most often associated with its dark past—a city defined by what happened to it, rather than what’s happening in it. Travelers routinely skip over the capital in favor of the Angkor Wat gateway of Siem Reap or neighboring tourism heavyweights like Bangkok and Ho Chi Minh. So...