Initial open source release with CC BY-NC-SA 4.0 license

- Add complete CC BY-NC-SA 4.0 International License
- Add comprehensive README with project description and usage
- Add CONTRIBUTING guidelines for developers
- Add Nix development environment with OpenSCAD tooling
- Add .gitignore for OpenSCAD and Nix projects
- Add copyright headers to source files

CONTRIBUTING.md

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+# Contributing to Cuatro
+
+Thank you for your interest in contributing to Cuatro! This document provides guidelines for contributing to this 3D printable game pieces project.
+
+## Ways to Contribute
+
+### Reporting Bugs
+- Check if the bug has already been reported in the issues
+- Provide clear steps to reproduce the issue
+- Include OpenSCAD version and operating system information
+- Attach screenshots or error messages if applicable
+
+### Suggesting Enhancements
+- Check existing issues for similar suggestions
+- Clearly describe the proposed feature or improvement
+- Explain the use case and benefits
+- Consider backward compatibility with existing designs
+
+### Code Contributions
+We welcome improvements to the OpenSCAD code! Areas where contributions are especially valuable:
+
+- **Parameterization**: Making more aspects of the design configurable
+- **New Piece Types**: Adding additional game piece variants
+- **Optimization**: Improving print efficiency or material usage
+- **Documentation**: Adding code comments and improving clarity
+- **Testing**: Validating designs across different print settings
+
+## Development Guidelines
+
+### OpenSCAD Code Style
+- Use descriptive variable and module names
+- Keep modules focused on single responsibilities  
+- Add comments for complex calculations or design decisions
+- Use consistent indentation (4 spaces)
+- Group related modules together
+
+### Design Principles
+- **Printability**: All designs must be 3D printer friendly
+- **Modularity**: Create reusable modules when possible
+- **Parameterization**: Use variables instead of magic numbers
+- **Efficiency**: Minimize material usage while maintaining strength
+- **Compatibility**: Maintain consistent scaling relationships
+
+### File Organization
+- Keep the main design in `parts.scad`
+- Use meaningful module names that describe their function
+- Place configurable parameters at the top of the file
+- Document parameter units and expected ranges
+
+## Submission Process
+
+### Before Submitting
+1. Test your changes in OpenSCAD (both preview and render)
+2. Verify the generated STL files are printable
+3. Check that existing functionality still works
+4. Update documentation if you've added new features
+5. Ensure your changes follow the project's design principles
+
+### Creating a Pull Request
+1. Fork the repository
+2. Create a descriptive branch name (e.g., `feature/hollow-star-pieces`)
+3. Make your changes with clear, focused commits
+4. Test thoroughly with different parameter values
+5. Submit a pull request with:
+   - Clear description of changes made
+   - Explanation of the problem solved or feature added
+   - Screenshots or images of printed results (if available)
+   - Any breaking changes clearly noted
+
+### Commit Messages
+- Use clear, descriptive commit messages
+- Start with a verb (Add, Fix, Update, Remove, etc.)
+- Keep the first line under 72 characters
+- Example: "Add hollow star piece variant with configurable points"
+
+## Testing Guidelines
+
+### Design Validation
+- Preview design in OpenSCAD (F5) for quick validation
+- Full render (F6) for final verification
+- Export STL and check for printability issues
+- Test with different parameter values to ensure robustness
+
+### Print Testing
+While not required for all contributions, physical print testing is highly valued:
+- Test with common 3D printer settings
+- Verify dimensional accuracy
+- Check structural integrity
+- Document any print-specific observations
+
+## Questions and Support
+
+- **Design Questions**: Open an issue for design-related discussions
+- **Technical Issues**: Include OpenSCAD version and code snippets
+- **Feature Requests**: Describe the use case and expected behavior
+
+## License Agreement
+
+By contributing to Cuatro, you agree that your contributions will be licensed under the same CC BY-NC-SA 4.0 International License that covers the project. You retain copyright of your contributions while granting the project rights to use and distribute them under this license.
+
+## Recognition
+
+Contributors will be acknowledged in the project documentation. Significant contributions may be recognized in release notes and project history.
+
+Thank you for helping make Cuatro better!