`export let` variable doesn't require the variable to be assigned within current file

### 🔎 Search Terms

`export let`, `undefined`, assignment, assigned

### 🕗 Version & Regression Information

- This is the behavior in every version I tried, and I reviewed the FAQ for entries about `export let`

### ⏯ Playground Link

_No response_

### 💻 Code

```ts
// a.ts
export let x: { foo(): void }

if (!!false) {
// @ts-expect-error Variable 'x' is used before being assigned.
x.foo()
}

// b.ts
import { x } from './a.ts'

// TS compiles, throws at runtime
x.foo()
```

### 🙁 Actual behavior

TS compiles, only throws at runtime.

### 🙂 Expected behavior

TS refuses to compile without `!` non-null assertion, with error probably being reported in the exporting module `a.ts` rather than the importing module `b.ts`.

### Additional information about the issue

_No response_

Start with the cross-file example in a.ts and b.ts and reproduce the current compilation and runtime behavior. Trace the definite-assignment check for the exported x and its use after import; done means the compiler reports the missing assignment rather than allowing the runtime failure.

Written by the indexing model from the issue text.