React 19 and Next.js 16 performance optimization for the Arcanea platform. Use when writing, reviewing, or refactoring React components, data fetching logic, bundle optimization, or any frontend performance work. Triggers on: React components, Next.js pages, hooks, data fetching, bundle size, re-renders, Server Components, Client Components, hydration. Sourced from Vercel Engineering's official React best practices (57 rules, 8 categories) and adapted for the Arcanea stack.