Cards on a long-lived branch no longer freeze Obeya

Before a card starts, Obeya checks what every card in progress has changed, to spot likely merge conflicts. It used to measure each card against main and read them one after the other while the server waited. So cards built on a long-lived branch counted that whole branch as their own work, and the server stopped answering on every canvas, for minutes on large repositories. Now each card counts only its own work, from where it left the branch it is built on, and the reading runs in the background while Obeya keeps answering.