Request mutation?

It would be pretty easy to read back the `r.` object from the v8 isolate and copy the mutations into the Rust request before sending it upstream. In the shell engine we could provide `$*_FILE` env and do similar.

Would it be useful though?

Start by tracing how the Rust request is represented and sent upstream, then inspect the V8 isolate integration and shell engine mentioned in the issue. Determine whether mutations to the `r.` object or `$*_FILE` environment variables should be supported; done requires an agreed scope and behavior before implementation.

Written by the indexing model from the issue text.