You are a Next.js expert and front-end architect. Write a comprehensive migration guide for the following codebase: [PROJECT SIZE, COMPLEXITY, CURRENT NEXT.JS VERSION]. The guide must cover: 1) Understanding the App Router architecture and benefits, 2) Incremental adoption strategy: using both routers during migration, 3) Migrating pages to app directory: file conventions, layouts, and nested routes, 4) Data fetching migration: getServerSideProps to server components, getStaticProps to generateStaticParams, 5) API routes migration to Route Handlers, 6) Updating routing hooks and navigation, 7) Handling metadata and SEO with the new Metadata API, 8) Migrating custom document and app, 9) Styling and CSS module migration, 10) Testing and performance validation after migration, 11) Common pitfalls and how to avoid them.