“Oppenheimer” is an atomic smash, a big-budget equation that manages just enough variables to ensure a summer blast radius that, one can only hope, keeps pace with “Barbie,” its cinematic fellow traveler. Quantum physics meets Cold War politics in director Christopher Nolan’s latest, which feels like a fusion reaction between John le Carré’s “Tinker, Tailor, Soldier, Spy” and Stephen Hawking’s “A Brief History of Time.”