Modernize tooling, require React 18 and fix cache boundary bugs

[devmaxxx]

Summary

Audit of the package: tooling brought up to date, React 18 made the minimum, the bugs below fixed, and an end-to-end suite added.

Tooling

• microbundle-crl → tsdown: ESM + CJS with type declarations through exports, checked by publint.
• yarn → pnpm workspace; the CRA example is now a Vite app that imports the built package.
• react-scripts/Jest → Vitest; new Playwright e2e suite against the example in Chromium, Firefox and WebKit.
• ESLint 6 → ESLint 10 flat config; Travis → GitHub Actions (React 18.0 / 18.3 / 19 unit matrix, CLI on Node 18–24, e2e).
• html-validate checks the source and the built index.html.

Bug fixes

• generate-build-meta crashed (mkdirp was never a dependency), had no shebang, and exited 0 on failure. It now uses only Node built-ins (no runtime dependencies left) and supersedes the mkdirp fix from Fix: fix mkdirp error #5.
• Focus and every state update started a new polling interval without clearing the old one; polling kept running while the window was blurred and ignored later duration/auto changes.
• clearCacheAndReload stored the old version, and a meta file without version reloaded forever with auto.
• Without auto, the new version was stored before the user reloaded, so a plain reload hid the update.
• An error thrown while loading was swallowed; errors now reach the parent boundary through getDerivedStateFromError.
• The meta file was always fetched from the site root; ./, ../, absolute paths and URLs are honoured, and an SPA fallback answering meta.json with HTML is detected instead of parsed.
• lazyRetry(fn, 0) retried forever and made one attempt fewer than asked.
• Error checkers built from a /g regexp answered differently on each call.
• Stale-chunk errors not recognised before: Vite CSS preload, native import() in Chrome/Firefox/Safari (including Safari's 'text/html' is not a valid JavaScript MIME type when the chunk URL returns HTML — found by the WebKit e2e run), webpack CSS chunks.
• A throwing debug callback could leave the fallback up forever.

Breaking changes

• react >= 18 peer; the CLI needs Node.js >= 18.3.
• lazyRetry(fn, retries) now makes retries extra attempts (default: 3 attempts in total instead of 2).
• See "Migrating from 0.2" in the README.

Test plan

• pnpm test — lint + html-validate, build + publint, typecheck, 43 unit tests, 21 e2e tests (7 scenarios × 3 browsers)
• Unit tests on React 18.0.0, 18.3.1 and 19.3.1
• CLI on Node 18.20 with and without --destination, failure path exits 1
• CI green on this PR