Summary

Fixes a CJS-interop default-export bug in the writePreBuildLibPath codegen for shared modules.

The branch selected when a shared package has any named exports emits:

import * as __mfPrebuildNamespace from '<pkg>';
const __mfPrebuildExports = __mfPrebuildNamespace;
...
export default __mfPrebuildExports;   // ← the raw namespace object

But the three sibling branches in the same function — react/jsx-runtime, react/jsx-dev-runtime, and the fallback for packages with no named exports — all use namespace.default ?? namespace. This was inconsistent codegen and caused a real interop regression for consumers.

Impact

For any shared package that exports named symbols and has a CJS default via ESM interop (react, react-dom, styled-components, react-router, etc.), import Pkg from 'pkg' through the prebuild shim returned the module namespace object instead of the real default export.

Concretely, when a host prebundles a shared react this way, downstream consumers see Pkg.createElement as undefined because the default they got was { default: React, useState, useEffect, ... } rather than React itself. This tends to surface as Cannot read properties of undefined (reading 'createElement') from prebundled shared React.

Fix

Emit __mfPrebuildNamespace.default ?? __mfPrebuildNamespace in the named-exports branch, matching the other three branches.

Tests

Added a regression test in src/virtualModules/__tests__/virtualShared_preBuild.test.ts that verifies the emitted code for a package with named exports (a) contains export default __mfPrebuildNamespace.default ?? __mfPrebuildNamespace; and (b) does not emit the old export default __mfPrebuildExports; line.

• pnpm test — 598/598 passing (was 597/597; +1 new test).
• pnpm fmt.check / pnpm typecheck clean.
• Verified the new test fails against main without the source fix and passes with it.