fixes #519 /claim #519

Hi, I have completed the schema migration module following the mentor’s requirements.

Features Implemented: Pure Data: Migrations are serializable data, not runtime code. Zero Runtime Overhead: Used structural types for older versions. Compile-time Validation: Macros ensure type safety before running. Nested Migrations: Supports deep changes in Lists, Maps, and Enums. Offline Support: Added JSON serialization/deserialization.

All tests are passing with full coverage. Please review.