Comprehensive documentation for the new customer pre-order workflow:
- Added docs/PREORDER/README.md with process, entities, lifecycle, and workflow details
- Registered topic code PREO in docs/TOPIC_CODES.md
- Added PREORDER_ISSUES.md and PREORDER_TODO.md for tracking
- Updated README.md, DOMAIN_MODEL.md, MEASUREMENT.md, and topic folder lists to reference pre-orders
- Enhanced SCHEMA.md with pre-order entity clarifications and business logic
- Integrated pre-orders as a fourth main domain workflow