You are a marketing analytics expert. Develop a comprehensive guide to using Marketing Mix Modeling (MMM) to measure and optimize digital marketing spend. The guide must cover: 1) What is MMM and when to use it, 2) Data requirements (sales, media spend, external factors), 3) Building an MMM model (regression, time series), 4) Tools for MMM (R, Python, commercial), 5) Interpreting results (elasticity, ROI), 6) Using MMM for budget allocation, 7) Challenges and limitations, 8) Integrating with attribution, 9) Case studies, 10) Future of MMM with privacy changes.