The Balboa Park Upper Yard, a wedge-shaped parking lot squeezed between Highway 280 and San Jose Avenue, isn’t anybody’s idea of picturesque real estate. But the wind-swept island of concrete could be the key to something that has confounded housing advocates for decades: how to turn one of San Francisco’s busiest transportation nodes from transit backwater to transit village In December, the Mayor’s Office of Housing and Community Development will issue a request for qualification for a developer to build 80 family housing units on the 1.8-acre property, which serves not only as a place to park automobiles but also as a “kiss-and-ride” drop-off point for commuters. The project, which could break ground in a year, will be 100 percent affordable, probably geared toward families earning 60 percent of area median income, about $61,000 for a family of four, said Teresa Yanga, director of housing development for the office. Broader interest sought But Supervisor John Aval