“Thursday morning in New York, Sotheby’s auction halls will be the stage for a historic struggle, the final chapter in one of the more fascinating and tortuous negotiations between a private collector and his courting museums. The Dundas Collection of Northwest Coast American Indian Art is up for grabs,” and Canadian museums want to see the works come home. But the collection’s owner has been playing coy with the Canadians and others for the better part of two decades, and no one really knows where the Dundas is likely to end up by week’s end.