- Add VariableSpec class with magnitude field validation
- Add Formula class supporting multiple input/output variables
- Support d4rt_code as string or object with code field
- Add comprehensive tests for parsing and serialization
- Fix broken test import in pruebas_d4rt_test.dart
Follows README.md format requirements exactly
- Replace generic sample app description with comprehensive project overview
- Add detailed feature descriptions for formula search, computation, and sharing
- Document unit management and d4rt interpreter integration
- Include rich formula documentation with markdown and images support
- Maintain existing project structure documentation
