“How con­sis­tent­ly you type on a key­board and the way you use your smart­phone may one day tell your doc­tor — or some­one else, like your employ­er — if your brain is ailing. Right now, doc­tors gen­er­al­ly rely on ques­tion­naires about a person’s symp­toms and writ­ten chal­lenges like draw­ing a clock to begin to home…