Add more schema documentation

Even though the schema is well-commented, it's still not describing some key design elements. Consider either embedding more documentation into the schema itself, or writing a separate document. Particularly including:

* Test classification tree
* Summarizing test runs
* Avoiding record duplicates
* Dealing with duplicates

Start by reviewing the existing schema comments and any separate documentation to identify the current coverage of test classification, test-run summaries, and duplicate handling. Done means documenting these four design elements clearly, either in the schema or a separate document, without leaving the intended approach ambiguous.

Written by the indexing model from the issue text.