Maybe a letdown — or at least a left turn — was inevitable. A week after Elim Chan made her high-impact debut as music director designate of the San Francisco Symphony, Chinese New Zealand guest conductor Tianyi Lu led an ill-conceived, overstuffed program at Davies Symphony Hall. “Scheherazade”: San Francisco Symphony. 7:30 p.m. Saturday, June 13; 2 p.m. Sunday, June 14. $55-$189. Davies Symphony Hall, 201 Van Ness Ave, S.F. 415-864-6000.