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
 ```

AGENTS.md

@@ -14,7 +14,7 @@ Also supports User-Agent Client Hints via an optional `clientHints` parameter on
 
 ## Commands
 
-- **Build**: `npm run build` (Webpack + Babel → `es5.js`, `bundled.js`)
+- **Build**: `npm run build` (tsdown + Babel → UMD `es5.js` / `bundled.js` and ESM `bowser.mjs`, all ES5; see `tsdown.config.ts`)
 - **Test**: `npm test` (AVA — runs both `test/unit/` and `test/acceptance/`)
 - **Lint**: `npm run lint:check` / `npm run lint:fix`
 
@@ -72,7 +72,7 @@ Returns `true`/`false` if the browser matches, or `undefined` if the browser isn
 
 ## Testing Details
 
-- **Acceptance tests** (`test/acceptance/test-list-of-ua.js`): Loads `useragentstrings.yml`, tests every UA against both `src/bowser.js` (ES6) and `es5.js` (built) — so **build before running tests** if you changed source.
+- **Acceptance tests** (`test/acceptance/test-list-of-ua.js`): Loads `useragentstrings.yml`, tests every UA against `src/bowser.js` and all three built artifacts (`es5.js`, `bundled.js`, `bowser.mjs`) — so **build before running tests** if you changed source.
 - **Unit tests** (`test/unit/`): Test individual Parser/Bowser/Utils methods. Use `new Parser(UA, true)` (skipParsing=true) for isolated method testing.
 - Test framework: AVA with Babel transpilation via `@babel/register`.
 

README.md

@@ -39,8 +39,9 @@ import * as Bowser from "bowser"; // TypeScript
 import Bowser from "bowser"; // ES6 (and TypeScript with --esModuleInterop enabled)
 ```
 
-The ES module build also exposes `parse` and `getParser` as named exports, so
-you can import just the part you use and let your bundler drop the rest:
+The ES module build also exposes `parse` and `getParser` as named exports.
+Detection needs every parser either way, so named imports are a matter of
+style, not bundle size — but an import that ends up unused is dropped entirely:
 
 ```javascript
 import { getParser, parse } from "bowser";

build/entries/bundled.js

@@ -11,6 +11,8 @@
  * `core-js/es` rather than `core-js/stable`: `stable` is `es` plus every web
  * polyfill, and the extra ones — `URL`, `URLSearchParams`, `queueMicrotask` —
  * were never in any released `bundled.js`, yet cost ~20 kB (~7 kB gzipped).
+ * The few `es` modules `@babel/polyfill` did not have are excluded in
+ * `tsdown.config.ts` (`NOT_IN_BABEL_POLYFILL`).
  * `test/acceptance/test-es5-runtime.js` pins the `@babel/polyfill` surface.
  *
  * See `./es5.js` for why this only re-exports the default.

test/acceptance/test-es5-runtime.js

@@ -179,3 +179,26 @@ test('bundled.js still installs everything @babel/polyfill did', (t) => {
   vm.runInContext(fs.readFileSync(path.join(root, 'bundled.js'), 'utf8'), context);
   t.deepEqual(BABEL_POLYFILL_SURFACE.filter((ref) => !present(ref)), [], 'bundled.js no longer installs these');
 });
+
+/**
+ * The other side of the contract: polyfills no released `bundled.js` ever
+ * shipped stay out, because each one is pure weight for existing consumers.
+ * Adding one back should be a deliberate decision that updates this list, not
+ * a side effect of switching a core-js entry point.
+ */
+const NEVER_SHIPPED = [
+  'this.URL', 'this.URLSearchParams', 'this.queueMicrotask',
+  'Array.prototype.flat', 'Object.fromEntries', 'NodeList.prototype.forEach',
+];
+
+test('bundled.js does not install polyfills no release ever shipped', (t) => {
+  const context = createEs5Context();
+  vm.runInContext(NEVER_SHIPPED
+    .map((ref) => `try { delete ${ref}; } catch (e) {}`)
+    .join('\n'), context);
+  vm.runInContext('this.NodeList = function NodeList() {};', context);
+  const present = (ref) => vm.runInContext(`typeof (${ref})`, context) !== 'undefined';
+
+  vm.runInContext(fs.readFileSync(path.join(root, 'bundled.js'), 'utf8'), context);
+  t.deepEqual(NEVER_SHIPPED.filter(present), []);
+});

tsdown.config.ts

@@ -38,11 +38,34 @@ const legacyBabel = (useBuiltIns: false | 'entry') => babel({
     modules: false,
     loose: true,
     useBuiltIns,
-    ...(useBuiltIns ? { corejs: '3' } : {}),
+    ...(useBuiltIns ? { corejs: '3', exclude: NOT_IN_BABEL_POLYFILL } : {}),
     targets: legacyTargets,
   }]],
 });
 
+/**
+ * The only core-js 3.0 `es` / web modules with no counterpart in the
+ * `@babel/polyfill` (core-js 2) set that every released `bundled.js` shipped:
+ * the `concat`/`splice` species fixes, `Array#flat`, `Object.fromEntries`, the
+ * Annex B `__defineGetter__` family and `NodeList#forEach`. Excluding them makes
+ * the payload exactly that set again, 2.8 kB (0.9 kB gzipped) smaller.
+ *
+ * babel rejects unknown names here, so a rename in core-js fails the build
+ * rather than silently shipping the module again.
+ */
+const NOT_IN_BABEL_POLYFILL = [
+  'es.array.concat',
+  'es.array.flat',
+  'es.array.splice',
+  'es.array.unscopables.flat',
+  'es.object.define-getter',
+  'es.object.define-setter',
+  'es.object.from-entries',
+  'es.object.lookup-getter',
+  'es.object.lookup-setter',
+  'web.dom-collections.for-each',
+];
+
 /**
  * Lowers the *emitted chunk* to ES5, after bundling and before terser.
  *
@@ -108,6 +131,8 @@ const terser = () => ({
       // IE 8: reserved words as property names must stay quoted.
       ie8: true,
       safari10: true,
+      // Output stops shrinking at 5 passes (measured; 6+ is byte-identical).
+      compress: { passes: 5 },
       format: {
         // Keep the `/*!` banner.
         comments: /^!/,