An update document may be an *array* of aggregation stages instead of a
document of operators. The pinned crud corpus has five cases, one per command,
and not one of them is a refusal -- so which stages are allowed, what happens
to `_id`, and what an upsert does are all unmeasured there.
23 cases into the existing operator corpus rather than an eighth directory:
this is another shape of update document, and it shares the recorder, the
masking and the README.
Recorded red, 0/23. What it settled:
- **`_id` always survives**, through `$replaceRoot: {newRoot: "$t"}` and
through `$project: {_id: 0}` alike. A stage that sets it to a *different*
value is ImmutableField (66); restating the same one is fine.
- `$match`, `$group`, `$sort` and `$unwind` are real stages refused
specifically here: InvalidOptions (72), "$X is not allowed to be used
within an update". A name that is no stage at all is 40324, and two
stages packed into one array element is 40323.
- `arrayFilters` beside a pipeline is FailedToParse (9).
- an upsert runs the pipeline over the document the filter implies.
Three shapes were authored and removed: `{b: 1, $set: {...}}`, an empty
pipeline, and a pipeline holding a non-document. The driver rejects all three
before they reach a server, so there is nothing to record.
