Add support for for-loops

Add support to inject progress updates in for loops. For reasons explained in , it's impossible to support:

```r
for (x in xs) {
sqrt(x)
} |> progressify()
```

but should be able to support:

```r
( for (x in xs) {
sqrt(x)
} ) |> progressify()
```

and

```r
{ for (x in xs) {
sqrt(x)
} } |> progressify()
```

We can also support it without pipes;

```r
progressify(for (x in xs) {
sqrt(x)
})
```

Start by locating the existing progressify implementation and how it handles piped expressions and function calls. Add coverage for parenthesized and braced for-loops, plus progressify(for (...) {...}), and verify that each form emits progress updates without supporting the explicitly excluded unwrapped pipe form.

Vom Indexierungsmodell aus dem Issue-Text verfasst.