Three years ago, Massachusetts brought near-universal health coverage to the state very quickly. To make it happen, Democratic lawmakers and Gov. Mitt Romney deferred until another day any serious effort to control the state's runaway health costs. But threatened first by rapid early enrollment in its new subsidized insurance program and now by a withering economy, the state's health overhaul has entered a second, more challenging phase. Due to new taxes and fees imposed last year, the health plan's jittery finances have stabilized for the moment. But government and industry officials agree that the plan will not be sustainable over the next 5 to 10 years if they do not take significant steps to arrest the growth of health spending.