React 19 in Production: What Actually Changed After Migrating aiPM and The Weekend Stores

I migrated two production Next.js 15 apps to React 19 — aiPM (multi-tenant SaaS with real-time AI streaming) and The Weekend Stores (e-commerce with complex cart state). Here's what the React Compiler actually did to bundle size and re-renders, where Actions simplified real form flows, and the one breaking change that cost me three hours.