It’s here. Almost. We hope. After nearly a century of development, the MTA says that Phase 1 of the Second Avenue Subway will be up and running by the end of December — bringing the Q line to a quartet of stations from 63rd to 96th streets. It’s optimistic, if belated, news from a city teasing the line’s promise since before the Great Depression. But this time there’s a sense that an end is finally in sight.