`flatten` accepts null/undefined type

The signature flatten(target: T, ...): R leaves T unconstrained, so TypeScript lets you pass `null | undefined`. However, at runtime the function expects an object and crashes.

Reproduction: https://playcode.io/2592924

Ideally this is prevented at compile time, by constraining T:
```
export function flatten>(
target: T,
options?: FlattenOptions
): R;
```

Related: https://github.com/hughsk/flat/pull/185

Start by locating the TypeScript declaration for the flatten entry point and inspect the signature that leaves T unconstrained. Use the reported Playcode reproduction or the repository's type-checking setup to verify that null and undefined are rejected while valid object targets remain accepted.

Written by the indexing model from the issue text.