This commit introduces several enhancements to the bitmap image processing library:

Added:
- New image manipulation functions:
    - Invert Colors: Inverts the RGB values of each pixel.
    - Sepia Tone: Applies a classic sepia filter.
    - Box Blur: Applies a configurable box blur effect.
- Comprehensive unit testing framework (`tests/test_bitmap.cpp`) using CMake/CTest.
- Unit tests covering:
    - Pixel-level operations (Greyscale, Invert, Sepia, Brightness, Contrast, Saturation, Luminance, including color-specific variants).
    - Image-level operations (BoxBlur, Shrink, Rotate, and basic validity checks for others).
- `README.md`: Details project features, dependencies, build instructions, and basic usage.
- `CHANGELOG.md`: Tracks changes, starting with this set of enhancements as v0.1.0.

Changed:
- Refined contrast adjustment logic for secondary colors (Magenta, Yellow, Cyan) to apply contrast directly to their constituent primary color channels for more predictable results.
- Significantly improved inline code documentation in `src/bitmap.h` and `src/bitmap.cpp` for all functions, enhancing clarity on purpose, parameters, and logic.
- Standardized clamping of pixel color components to the 0-255 range across various functions.
- Refactored `main.cpp` to serve as a clearer demonstration of library features, including new functions and better error handling for file operations.

Fixed:
- Corrected syntax errors in the original implementations of `ChangePixelContrastMagenta` and `ChangePixelContrastCyan`.
- Addressed incorrect variable usage (e.g., `magenta` vs `cyan`) in `ChangePixelContrastCyan`.
- Corrected `std::min` usage in several saturation and luminance functions.
- Improved GDI resource management in `ScreenShotWindow` by ensuring proper cleanup of device contexts and bitmap handles.
- Corrected minor typos in function parameter names and conditional logic in some pixel manipulation functions.