Biogen Inc. shares surged after its controversial Alzheimer’s disease therapy was approved by U.S. regulators, a landmark decision that stands to dramatically change treatment for the debilitating brain condition. The stock gained as much as 64% shortly after the news of the clearance, triggering a brief volatility halt. It ended up 38% at $395.85, the biggest single-day gain since drug officials first said that the treatment appeared effective in November.