What are the key considerations in gravel bike frame design that influence rider position, and how do these design elements impact comfort, efficiency, and overall performance on variable terrain? Specifically, how do factors such as head tube angle, seat tube angle, and bottom bracket drop interact to create an optimal riding position for gravel biking, and what trade-offs might be necessary to prioritize comfort over performance or vice versa? Are there any emerging trends or innovations in gravel bike frame design that are redefining the relationship between rider position and bike handling, and if so, what implications do these developments have for riders seeking to optimize their gravel biking experience?