Add regression test confirming visual-scenes-500.jsonl satisfies issue #600:
- 500 valid JSONL records
- Required fields (terse, rich, domain) all present and non-empty
- Domain equals "visual scenes" for every record
- Full-record uniqueness
This closes the loop on Training Factory Worker 1/6 (visual scenes).
The dataset was originally added via PR #731 (merged to main).
Closes#600.