LOS ANGELES — Rain began falling in parched Southern California on Saturday in a boon for firefighters who were mopping up multiple wildfires. But heavy downpours on charred hillsides could bring the threat of new troubles like toxic ash runoff. Los Angeles County crews spent much of the week removing vegetation, shoring up slopes and reinforcing roads in devastated areas of the Palisades and Eaton fires. The two fires reduced entire neighborhoods to rubble and ash after breaking out during powerful winds on January 7. Most of the region was expected to get about 2.5 centimeters of precipitation over several