Americans might be counting on the day when home and retirement-fund values start to rise again, but anyone expecting to benefit from a future boom in prices should take note: Economic policymakers around the world are looking for ways to make sure that doesn’t happen, or at least not with such intensity that it risks the kind of bust that usually follows. In studying how to respond to the recent crisis and create a more stable system, central bankers, international officials and others have been focusing on a concept known as “systemic risk.”