BAGHDAD — Protesters stormed Iraq’s parliament Saturday in a dramatic culmination of months of demonstrations, casting uncertainty over the tenure of the country’s premier and the foundations of the political system laid in place after the 2003 U.S.-led invasion. Security forces declared a state of emergency in the Iraqi capital after demonstrators climbed over blast walls and broke through cordons to enter Baghdad’s fortified Green Zone, also home to ministries and the U.S.