marshal: Rcpp::sourceCpp()

From https://future.futureverse.org/articles/future-4-non-exportable-objects.html#package-rcpp:

```r
Rcpp::sourceCpp(code = "
#include
using namespace Rcpp;

// [[Rcpp::export]]
int my_length(NumericVector x) {
return x.size();
}
")
```

This produces:

```r
my_length()
```

which holds an external reference pointer.

Can such an object be marshalled?

Start with the Rcpp::sourceCpp() example in the issue and inspect how marshal handles objects containing external reference pointers. Determine whether this object can be marshalled safely across R processes; document the result and any limitations or required scope.

Written by the indexing model from the issue text.