The biggest hedge fund startup so far this year is debuting with $2 billion in investor commitments and former Citadel traders Michael Rockefeller and Karl Kroeker at the helm. Their firm, Woodline Partners, will begin trading Thursday after the duo decided to cap the size of their fund, according to people familiar with the matter. They’ll be among a small group of well-pedigreed managers expected to open with at least $1 billion in 2019.