perf: trim bundled.js to the 2.14.1 polyfill set, fix stale docs

- Exclude the 10 core-js 3.0 modules with no counterpart in the
  @babel/polyfill set every released bundled.js shipped (concat/splice
  species fixes, Array#flat, Object.fromEntries, the __defineGetter__
  family, NodeList#forEach): 156.1 -> 153.2 kB (49.3 -> 48.4 kB gzip).
  babel rejects unknown exclude names, so a core-js rename fails the build.
- terser `passes: 5`, where output stops shrinking: es5.js 7.52 -> 7.49 kB
  gzip.
- New test: bundled.js must not install polyfills no release ever shipped
  (fails on the previous master build, passes on 2.14.1 and this one).
- README no longer claims named imports shrink the bundle; AGENTS.md
  describes the tsdown build and four-artifact acceptance tests;
  CONTRIBUTING points at the existing lint:check script.

.github/CONTRIBUTING.md

@@ -38,5 +38,5 @@ If you'd like to contribute a change to `bowser`, modify the files in `src/`, an
 $ npm install
 $ npm run build #build
 $ npm test #run tests
-$ npm run lint #check lint rules
+$ npm run lint:check #check lint rules
 ```