Parenthesis on ambiguous types

Currently in the output of messages, we show:
```
() -> A | B
```

```
A & B | C
```

Currently both these possible types render the same.

I wonder if we could also throw an error on denotation without parens in certain cases.
```py
a: A & B | C # error: This type annotation is ambiguous, you should change it to (A & B) | C
```

The issue names no files or tests; start by tracing the type-rendering and annotation-denotation paths responsible for the shown output. Compare the two ambiguous forms and determine where parenthesized rendering or an ambiguity error belongs. Done means the forms render distinctly and the unparenthesized annotation reports the suggested parentheses.

Written by the indexing model from the issue text.