Build process: Vite, Webpack and optimization

You are an engineer specializing in frontend tooling. Help me with the build: the project is [DESCRIBE: stack — React/Vue/TS?, size, current bundler — inherited Webpack/Vite/dead CRA/don't know], and the need is [NEED: migrate from Webpack-CRA to Vite, understand and optimize current build — heavy bundle, slow build —, configure from scratch correctly, resolve a specific build error — I can paste config/error]. Deliver: the non-tribal overview (what a bundler ACTUALLY DOES — resolve modules, transform, package, optimize: the mental map that makes any config make sense; tools in the right role — Vite as current standard for apps: instant native ESM dev server + Rollup in build; esbuild/SWC as the fast engines underneath; Webpack when legacy/specific plugin ties you down — no drama; the verdict for my case), migration to Vite when it's the case (the real step-by-step from your setup: index.html to root, env vars — REACT_APP_ to VITE_ and import.meta.env, aliases reproduced, dev proxy, asset imports, plugin for each need — the list of equivalents for my Webpack loaders/plugins; typical gotchas — dynamic require, process.env, stubborn CommonJS — with solutions), bundle slimmed down with method (the bundle ANALYZED before touching it — rollup-plugin-visualizer/webpack-bundle-analyzer: the weight map; code splitting by route with dynamic import — manualChunks/splitChunks separating vendor from what changes; tree shaking actually working — named import instead of giant default, lodash-es instead of lodash, sideEffects declared; heavy lib confronted — moment→date-fns/dayjs and such: the swaps for MY bundle; honest polyfill and target — browserslist for who I REALLY serve), complete production build (minification, source maps in production — the decision: hidden for Sentry without exposing; hash in name for infinite cache; env by environment without secrets in bundle — what's public by definition in frontend), sharp dev experience (real HMR, TypeScript checking in parallel without freezing reload — vite-plugin-checker, local API proxy), healthy CI build (dependency caching, deterministic build with lockfile, bundle size limit as gate — size-limit screaming in the PR that got fat), and if you paste config/error: line-by-line reviewed surgery. Goal: the dev server that opens in 1 second, the build that CI finishes fast — and the bundle that the user downloads without suffering.