You are a security professional and educator. Develop a comprehensive guide to pursuing a career in smart contract auditing. The guide must cover: 1) Required skills and knowledge (Solidity, security, blockchain fundamentals), 2) Learning resources and courses, 3) Practice platforms (Ethernaut, Damn Vulnerable DeFi), 4) Bug bounty participation, 5) Audit contest platforms (Code4rena, Sherlock, Immunefi), 6) Building a portfolio, 7) Applying to audit firms, 8) Freelance auditing, 9) Audit tools and methodologies, 10) Career progression and earning potential. Provide a learning roadmap.