Add Nexus Chaos sandbox game

Single-file interactive HTML sandbox at game/index.html that mirrors
PaymentProcessingWorkflow over the compliance-endpoint Nexus call.
Stop the Payments / Nexus / Compliance services and watch in-flight
workflows pend (not fail) until the service returns.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: nadvolod