Invalid syntax can pass Validate if it's in part of an expression that is not evaluated

This limitation is described in the README at https://github.com/getodk/validate#how-validate-validates but I think it's worth calling out in an issue as well.

https://github.com/enketo/enketo/issues/1097 has another example of a form that passes Validate but has bad behavior because of an `or or`.

Start with the README section on how Validate validates and compare its limitation with the example linked from enketo/enketo#1097. Trace how an expression containing invalid syntax such as `or or` is handled; done should be defined by an agreed resolution for forms that currently pass Validate but behave incorrectly.

Written by the indexing model from the issue text.