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.
