Creating the Game's Library

# Library Implementation

## The following header files are incomplete

- [x] `race.hpp`
- [x] `item.hpp`
- [x] `entity.hpp`
- [ ] `region.hpp`
- [ ] `world.hpp`

## The following source files are incomplete

- [x] `race.cpp`
- [ ] `item.cpp`
- [x] `entity.cpp`
- [ ] `region.cpp`
- [ ] `world.cpp`
- [x] `cartesian.cpp`
- [ ] `euclidean.cpp`

### Notes

- Contact @Irvinwop with questions about implementation.

The issue lists incomplete header and source files: region.hpp, world.hpp, item.cpp, region.cpp, world.cpp, and euclidean.cpp. Start by examining the existing, completed files (race.hpp, race.cpp, entity.hpp, entity.cpp, cartesian.cpp) to understand the project's structure and coding patterns. Contact @Irvinwop for implementation details, as the notes suggest. 'Done' means implementing the missing files to match the project's design.

由索引模型根據 Issue 內容生成。