CJS interop declaration transform produces invalid .d.cts when file has type-only named exports

When a source file has type-only named exports (export type, export interface, export enum) alongside export default, the CJS interop declaration transformer rewrites export default → export = but leaves the type-only exports with their export keyword. This produces an invalid .d.cts:

export type Foo = { bar: string };
export = MyLib;  // TS2309: An export assignment cannot be used in a module with other exported elements. Consumers with skipLibCheck: false see TS2309 on every such file.

Reproduction

Any source file with this common pattern:

export type Foo = { bar: string };
export interface Bar { qux: number }
type MyLib = { (input: Foo): Bar };
export default MyLib;

Expected behavior

The .d.cts should either:

Keep export default (valid when esModuleInterop is enabled), or
Not apply the CJS interop transform at all for files with type-only exports

Actual behavior

The .d.cts has export = mixed with export type / export interface, which is invalid TypeScript.