singular assignments in functions should become `Final`

```py
def f():
a = 1
b: 1 = a # error
```

This shouldn't be an error, `a` can never be modified.

No source files or tests are named. Start by reproducing the shown function and tracing the type-checker path for a single assignment with an annotation inside a function; done means the example is accepted while genuinely mutable assignments remain checked appropriately.

Written by the indexing model from the issue text.