What is module augmentation?

Question presented to candidate: "Your auth middleware puts a user on every request object, but the framework's Request type does not have it. How do you add that property to a third-party library's types without forking them, and what can go wrong?"

What a strong answer should cover:

• Module augmentation: declare module "lib" { interface Request { user?: User } } inside a file that is itself a module merges new members into the library's exported interface
• It is declaration merging across a module boundary, so you can extend interfaces (and namespaces), not type aliases; libraries expose interfaces like Vue's ComponentCustomProperties precisely to be augmented
• The file must be a module (a top-level import/export): in a script file the same block is an ambient module declaration that replaces the library's types
• Global augmentation: declare global { interface Window { ... } interface Array<T> { ... } } from a module, the same mechanism for global types
• Augmentation is type-only: it never adds the run-time value, so the middleware, plugin or polyfill must really provide it; export default x inside an augmentation is TS2666
• Placement: the augmenting file must be part of the compilation (include, or imported), otherwise the extra members silently disappear

Clarifying questions expected:

• Is the property set on every request, or only after a specific middleware (should it be optional)?
• Is this app code, or a library/plugin that should ship the augmentation to its users?

Code / implementation expected: Yes — a declare module block that adds a property to a library interface, and a usage that type-checks.