SwiftData Migration Writer

Create migration from UserDefaults to SwiftData:

1. Map keys to entity fields

2. Write migration method:

   func migrateXIfNeeded() async throws {
       guard !hasMigrated("X") else { return }
       // Read UserDefaults
       // Create/update entity
       // Archive old keys
       // Mark migrated
   }
   

3. Add to PreferencesStore extension

4. Call on first load

5. Test data preservation