Global variables

```c
int z;

void f () { z = 0; }
```
Gives:
```fstar
fn f ()
{
z := 0l;
}
```

With a non-existing `z`. At least, we should properly raise a translation error if we find references to global mutable variables.

We should discuss how to actually support it.

No source file, test, or entry point is named. Start by locating the C-to-F* translation path that handles references to global variables, then reproduce the example and trace how the undeclared z is emitted. Done means the invalid global mutable reference produces a translation error; broader support remains a separate design question.

Written by the indexing model from the issue text.