test: guard ES5 runtime APIs and type-check consumers in CI

Follow-up to the ES5 syntax fix, closing the gaps that investigation left.

An ES5-only runtime sandbox. The acorn check catches syntax, but syntax is
only half the contract: preset-env lowers syntax and never polyfills library
calls, so one `Array.prototype.includes` in the parser source compiles
cleanly, passes every test on modern Node, and throws on the browsers es5.js
exists for. The new test runs both legacy bundles in a vm context with the
post-ES5.1 globals, statics and prototype methods deleted, and asserts
bundled.js additionally installs the polyfills its README entry promises.
Includes a test that the sandbox really strips, so it cannot quietly pass
against a modern global.

A consumer type-check across every module resolution mode, run in CI against
the packed tarball. attw already checks that types *resolve* per condition;
it compiles nothing, so it cannot catch a declaration that resolves correctly
and then misdescribes the runtime. Negative cases are asserted too — the maps
must stay non-importable as named exports, which is the line index.d.mts
draws deliberately and only a failing compile can hold.

Also documents two findings that were investigated and deliberately left
alone: the bundled.js size increase is the core-js 2 -> 3 upgrade rather than
waste, and `useBuiltIns: 'usage'` would shrink it by breaking the documented
"all needed polyfills" contract; and the src/*.js ESM-in-CJS wart (publint
warnings, Yarn PnP, Node < 20.19) is longstanding and identical on 2.14.1,
with the nested-package.json fix blocked on @babel/register.

Verified by breaking each guard in turn: an ES6 API call injected into es5.js
fails the sandbox test, and an index.d.mts with its `parse` export removed
fails all three ESM resolution modes while the CJS modes correctly still pass.

tsdown.config.ts

@@ -23,6 +23,17 @@ const legacyTargets = {
 /**
  * `useBuiltIns: false` for `es5.js` (syntax transpilation only) and `'entry'`
  * for `bundled.js`, which expands the `core-js/stable` import in its entry.
+ *
+ * `'entry'` is why `bundled.js` grew from 124 kB to 174 kB when it stopped
+ * being built from the deprecated `@babel/polyfill`. That package was core-js
+ * **2**; `core-js/stable` is core-js **3**, whose stable surface is genuinely
+ * larger — `globalThis`, `Object.fromEntries` and `URLSearchParams` are all
+ * new here. The extra weight is the upgrade, not waste.
+ *
+ * Switching to `useBuiltIns: 'usage'` would shrink the bundle a long way, and
+ * would be wrong: the README tells consumers to reach for `bundled.js`
+ * precisely when they have no polyfills of their own, so it has to keep
+ * shipping the full payload rather than only what bowser itself calls.
  */
 const legacyBabel = (useBuiltIns: false | 'entry') => babel({
   presets: [['@babel/preset-env', {