David Muir has emerged as broadcast television's star of the summer.The “World News Tonight” anchor's nightly summary averaged 8.2 million viewers last week, more than any prime-time program on television, the Nielsen company said.ABC's prime-time interview of the Democratic ticket of Joe Biden and Kamala Harris, conducted by Muir and Robin Roberts, reached 5.1 million viewers Sunday.Otherwise, coverage of the Democratic National Convention on MSNBC and CNN dominated the ratings.CBS' prime time