Export stubs whose names aren't identifiers

[rosston]

This fixes the stub module quibble generates for a mock, so any export name works. Before, on Node 24+, td.replaceEsm('some-cjs-package') failed with SyntaxError: Unexpected token '.'. The stub couldn't handle export names that aren't JavaScript identifiers, and Node 24+ gives every CommonJS module one called 'module.exports'.

Why it broke

When you mock an ES module, quibble generates the source of a stub module that exports your stubs. It declared each named export with export let ${name} = …, so the name had to be a valid identifier.

Node 24+ reports two exports when ES module code imports a CommonJS package: default and 'module.exports'. Node 22.14 and earlier report only default. When testdouble.js's td.replaceEsm() is given no replacement, it imitates the real module and passes all of its export names to quibble.esm(). The stub then contained export let module.exports = …, which is a syntax error.

Calling quibble.esm() directly with a name like 'kebab-case' broke the same way, on any Node version.

Why the fix works

JavaScript lets a module export a binding under any string name: export { local as "any name" }. The stub now binds each stub value to a local variable and exports that under its real name, as a string:

let __quibbleExport0 = /* the stub for 'module.exports' */;
export { __quibbleExport0 as "module.exports" };

Importers see the real name, and the local name exists only inside the generated module. Each stub module has its own scope, so the locals can't clash across modules or mock generations. Because names are now JSON-encoded, quotes and backslashes in a name can't break the generated source either.

The change is all in transformModuleSource in lib/loader-helpers.js, which both the async hooks and the Node 26+ sync hooks use.

New tests

The change adds two tests in test/esm-lib/quibble-esm.test.mjs:

• Named exports whose names aren't identifiers ('module.exports', 'kebab-case', and one with quotes and backslashes). This fails without the fix on every Node version.
• A CommonJS module mocked with the export names Node gives it, the way td.replaceEsm() does. This fails without the fix on Node 24+.