Partitions sessions into N rooms, runs gamesPerRoom games per room
in parallel via Promise.allSettled so a failure in one room never
unwinds the others. Errors roll up into ScenarioResult.errors.
Verified via tsx: listScenarios() returns [populate], getScenario()
resolves by name and returns undefined for unknown names.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
