Kyle Shanahan paces around the sidelines at the best of times. He’s got Mike Hussey-itis in that sense.   In high-stress situations, you can see that he’s always looking for something to do with his hands, usually flicking his mic on and off. When he calls a good run, he struts. Even though the 49ers have kind of got away from classic Shanahan/Kubiak principles in recent years as Brock Purdy has matured into a truly good player on his own, you can feel at soul-level that Shanahan would prefer a 17-yard run to a 27-yard pass.   On Friday morning, on the very same MCG that Dustin Martin used to strut around when…