CBS led Wednesday by all measures, thanks to the 2-hour premiere of summer reality stalwart “Big Brother 18.” Compared to its year-ago premiere (“Big Brother 17” on June 24, 2015 8-9 p.m.), it was down by 10 percent (in total viewers; 6.19 million vs. BB17’s 6.92 million) and by 12 percent (in adults 18-49; 2.35 […]