BREAKING CHANGE: System metadata location changed from 'index/' to '_system/' directory
- Rename INDEX_DIR to SYSTEM_DIR following database conventions
- Implement dual-read/write strategy for zero-downtime migration
- Add automatic migration from old to new location on first access
- Support mixed service versions sharing S3/cloud storage
- Add 30-day grace period for gradual rollout (configurable)
- Store distributed config alongside statistics in _system folder
- Add comprehensive migration guide and documentation
Migration features:
- Read from both locations (new first, fallback to old)
- Write to both during migration period
- Automatic data migration when found only in old location
- Services can update independently without coordination
- Full backward compatibility for production deployments
The change improves clarity ('_system' better represents system metadata than 'index')
and follows standard database conventions (MongoDB's _system, PostgreSQL's pg_*).
