Director Costa-Gavras’ stock keeps slipping with wheezy melodramas like his latest, Mad City. But this career-high assassination thriller remains fresh as a news flash, even though the Greek-government conspiracy it dissected has faded to arcana for U.S. audiences. With a snazzy new transfer to show off all the swooping camera moves (clearly Oliver Stone and Steven Spielberg were taking notes) and improved subtitles that only occasionally lag behind the staccato testimony of multiple witnesses, Z remains a vital primer on thriller technique. A