The United States is open to fresh negotiations with Canada to resolve a longstanding dispute over U.S. tariffs on softwood lumber but a core sticking point remains, U.S. Trade Representative Katherine Tai said on Wednesday.  Tai told a U.S. Senate Appropriations subcommittee meeting that a new lumber trade agreement requires the Canadian government to address U.S. concerns that its policies amount to subsidies for Canadian producers.  "When and if Canadian industry and the Canadian government are ready to address those issues, we stand ready and willing to enter into negotiations to see ...