context: decide whether `undefined` should remain a valid ContextClient/ContextItem state

`ContextClient`'s class-level TSDoc has an open design question:

```ts
/**
* ...
* @todo - should this have `undefined` as a valid value?
*/
export class ContextClient extends Observable {
```

`ContextClient` (and `currentContext`) currently allows `ContextItem | null | undefined` as valid states. Decide whether `undefined` should remain a valid emitted/settable value (e.g. meaning "not yet initialized") or whether the API should be simplified to only `ContextItem | null`.

_Originally flagged as an `@todo` comment while resolving fusion-lint warnings on `fix/sematic-lint`._

Start by reading the ContextClient declaration and the currentContext API referenced in the issue, including their surrounding TSDoc and type usage. Determine whether undefined represents a necessary uninitialized state; done means the decision is documented and the ContextClient/currentContext contract consistently reflects it.

Written by the indexing model from the issue text.