Separate activeDid from monolithic settings table into dedicated
active_identity table to improve data normalization and reduce cache
drift. Implements phased migration with dual-write triggers and
fallback support during transition.
- Add migrations 003 (create table) and 004 (drop legacy column)
- Extend PlatformServiceMixin with new façade methods
- Add feature flags for controlled rollout
- Include comprehensive validation and error handling
- Maintain backward compatibility during transition phase
BREAKING CHANGE: Components should use $getActiveDid()/$setActiveDid()
instead of direct settings.activeDid access
