As President Trump doubles down on his tough talk against North Korea, tourists and residents in downtown Boston on Thursday said the international brinksmanship is making them uneasy — to say the least. On Tuesday, the president reacted to reports that North Korea had produced a compact nuclear warhead by warning that the country would face “fire and fury like the world has never seen’’ if it threatened the United States. And on Thursday, Trump said his “fire and fury’’ threat might not have been “tough enough.’’ If North Korea attacked the US or its allies, he said “things will happen to them like they never thought possible.’’ Passersby outside Faneuil Hall Thursday afternoon said the North Korea conflict was leaving them worried. Very few felt confident they knew how the whole ordeal would play out.