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.

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), []);
+});