World-weary detective falls for sphinx-like widow in a murder case. Talk about the usual suspects! We have seen this setup once or twice. But “Decision to Leave,” director and co-writer Park Chan-wook’s dazzling, confounding, gorgeously crafted variation on a dangerously familiar film trope, takes its component parts and comes up with something no one has ever built before. Visually it’s alive every second, in ways both considered and imaginative; the story, meanwhile, takes some risky wait-what? detours en route to a surprisingly grave finish. The South Korean genre master, whose films includ…