Verify each consumer history segment before opening a source editor.
Start new segments after external writes while retaining undo entries,
no-op baselines and recorded failures.
Add real-event controls for update-main, older undo/redo and failure
preservation. The original F250 regression passes with unchanged assertions.
Validation: 1,768 frontend tests, 15,004 assertions; lint and format pass.
This changes only the frontend test harness; no sync benchmark is needed.
AI-assisted-by: gpt-6
1732b730a2