Perched atop craggy bluffs in a beachside city north of San Diego, a railroad line offers passengers a sweeping view of the Pacific Coast. But the ground beneath it is crumbling. No one denies the problem, but a fight over how to solve it highlights a broader challenge, and a worrisome reality, for California residents: how to adapt to climate change that threatens coastal living, a way of life that has long defined the state’s identity, from its economy to its culture.