Massachusetts residents got a shock when state officials, at the peak of construction on the Big Dig project, disclosed that the price tag had ballooned to nearly $15 billion. But that, it turns out, was just the beginning. Now, three years after the official dedication of the Central Artery/Third Harbor Tunnel, the state is reeling under a legacy of debt left by the massive project. In all, the project will cost an additional $7 billion in interest, bringing the total to a staggering $22 billion, according to a Globe review of hundreds of pages of state documents. It will not...