Check result _value column not available for message body.

__Steps to reproduce:__
List the minimal actions needed to reproduce the behavior.

1. add { r._value } in check message body
2. save
3. view execution result

__Expected behavior:__
The check should populate the _value according with the results columns.

__Actual behavior:__
check execution fails due to {r._value}

Start with the reproduction steps: add { r._value } to a check message body, save it, and view the execution result. Trace how check results are exposed to message-body expressions and compare the available result columns; done means _value can be referenced without failing and the behavior is covered by a regression test.

Written by the indexing model from the issue text.