Validation for Notes field

Currently if we add any validation (required/constraint) to "Notes" field then it becomes impossible to submit the Form.

For example:

```

A readonly integer

```

### Questions:
1- Should engine restrict having readonly fields without any calculations and without any relevance to have required/constraint?
2- Should we show asterisk on the "required notes" field? Currently this happens on UI

From https://github.com/getodk/web-forms/pull/181:

> I'm not sure I'd necessarily want to see tests for this, but I will also say that it would be good for us to test somewhere in the stack some expectations around "weird forms". In this case, some areas I think about are where notes intersect with validation.

No files or tests are named. Start by reproducing the readonly integer form with required and constraint validation, then trace submission blocking and the required-field asterisk in the validation and UI paths. Done means the expected behavior for these fields is decided and covered for both submission and display.

Written by the indexing model from the issue text.