God bless Robert Downey Jr. As Tony Stark, the lonely tin man with a hole in his chest, Downey Jr. does more with his face than all of the Avengers films’ awkward speechifying about teamwork, solidarity and what it means to be Earth’s Mightiest Heroes. A decade of story-telling synergy comes to a close with Avengers: Endgame. So much of it is the sludge we’ve become used to, but when the thrusters kick in, and the film finds its heart, it’s because of Downey Jr. He almost — but not quite — makes all of this worth it. It’s been some time since the…