Credit the Legislature’s steady eroding of mayoral control, a union’s quest to protect outrageous perks and disarray at City Hall for a standoff that threatens to strand 150,000 city kids by week’s end. The proximate cause: Amalgamated Transportation Union 1181’s demand for employee-protection rules that violate state law. The union has convinced enough members of the Panel for Educational Policy, which oversees the city Department of Education, to refuse to renew yellow-bus contracts for the