In late September, Culver City became the first municipality in California to legalize the construction of mid-rise apartment buildings with a single staircase. Unless you’re a member of the niche but fervent subculture of architects, urbanists and pro-housing advocates who clamor for “single stair reform,” this might not sound like big news. But supporters say it could be the key to unleashing the kind of urban apartment building boom that years of pro-development legislation in Sacramento has