You are a UI designer. Develop a comprehensive guide to loading states. The guide must cover: 1) Types (spinners, progress bars, skeletons), 2) Skeleton screens vs spinners, 3) Placement and timing, 4) Animation design, 5) Skeletons for different components, 6) Full-page loading, 7) Content-aware loading, 8) Accessibility for loading, 9) Performance perception, 10) Design examples. Provide specs.