Feature Request: Nested Update

I want to be able to do something like:

```
update User
filter .id := $id
set {
email := $new_email,
billing_info: { email := $new_email }
}
```

Silly example but you get the idea. No idea if it's even feasible, but if it is.

The issue names no files, tests, or entry points; start by reviewing how update statements currently handle nested object fields and whether the shown syntax is feasible. Before implementation, define the supported nested-update semantics and acceptance cases, since the issue currently provides only an example and no precise completion criteria.

Written by the indexing model from the issue text.