The $35,000 Model 3 due to roll off the assembly line today is the most inexpensive vehicle Tesla has ever made. But nothing has been more valuable to the electric car company. As its stock has plunged, shedding more than $10 billion in market value over the past two weeks from a June 23 peak of $63 billion, Tesla’s hopes for the future are riding on its new mass-market vehicle, intended to vault it into