You are a UX designer. Develop a comprehensive guide to habit formation design. The guide must cover: 1) Hooked Model (trigger, action, reward, investment), 2) Building intrinsic motivation, 3) Variable rewards, 4) Onboarding for habit formation, 5) Retention strategies, 6) Ethical considerations (addiction), 7) Examples of habit-forming products, 8) Testing for habit strength, 9) Best practices, 10) Balancing engagement and well-being.