CHOTYIEL, South Sudan — It was 1 p.m., her children still hadn’t eaten, and every item on Nyaguey Dak Kieth’s “long to-do list” pertained to surviving another day. So Nyaguey grabbed a plastic bucket and an empty sack and set off from her village surrounded by floodwater. Those waters had upended her life, but also provided a food option — not a desirable one, but one of the few left.