[Bug]: all post-processing tests use canned data, not generated data

### Contact Details

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### Are there any linked Issues or Pull Requests?

_No response_

### What happened?

given that all of post-processing tests use canned data, not generated data, any changes to the output data format are not tested for post-processing compatibility

at last some tests should generate their own data using the current Vernier and then parse it, else we're only testing compatibility to the can

### Version

main

### Relevant log output

```shell

```

Start by locating the post-processing tests and the existing Vernier data-generation and parsing entry points. Replace or supplement canned-data coverage with tests that generate data using the current Vernier, then parse it; done means format changes are checked for post-processing compatibility.

Written by the indexing model from the issue text.