CAIRO — Seeking to reassure Egyptians and the world about its intention to return to civilian democracy, the military-led interim government on Tuesday laid out a brisk timetable to overhaul Egypt’s suspended Constitution, elect a new Parliament and choose a new president, all in the space of about six months. The release of the new timetable, issued in the name of the interim president, Adli Mansour, appeared intended to show steps toward civilian democracy after the military’s mass shooting of more than 50 Islamist protesters on Monday raised new doubts about the democratic promises of the generals who ousted former President Mohamed Morsi last week. Under United States law, if Washington officials deem the generals’ takeover to be a “coup” or decides that Cairo is moving away from democracy, then the Egyptian military stands to lose about $1.3 billion a year in American aid.