I’ve spent enough time in New York City government to know that real fiscal trouble rarely begins with a dramatic collapse. It starts with a warning: a number moved here, an outlook revised there, a signal from the bond market that traders suspect the adults may no longer be in charge. That’s what happened last week when Moody’s, a leading global provider of credit ratings and research,  revised the city’s credit outlook to negative, while S&P Global Rating, another of the industry’s “Big Three,