The Richmond branch of the American Association of University Women observed its 100th birthday yesterday with a party that included high school singers modeling clothes from each decade, a review of major news events in the organization’s lifetime and a look at the next century. Thomas A. Silvestri, publisher of the Richmond Times-Dispatch, led the historical flashback with a presentation of famous pages and headlines from the newspaper. Then Linda D. Hallman, national executive director of AAUW, spoke about the organization’s direction for the next century.