C++ code

# C++ code
- Use C++23 standard features (avoid modules).
- Follow the C++ Core Guidelines and modern best practices.
- Use smart pointers and RAII; avoid raw pointers unless absolutely necessary.
- Use `const` and `constexpr` wherever possible.
- Format code with consistent indentation: **use tabs** (1 tab = 4 spaces visually).
- Prefer brace initialization `{}` for variables and objects, except for inline member initializers where `=` is preferred.
- For member variables:
  - Use inline initialization with `=`.
  - End names with a trailing underscore (`score_`).
- Prefer `enum class` over traditional enums.
- Use classic header guards (`#ifndef`, `#define`, `#endif`) with guard names like `APP_GAME_INPUT_H` for `app::game` namespace in file `input.h`.
- Prefer `std::` library facilities over C-style code and avoid macros.
- Write clear, self-documenting code with meaningful names.
- For virtual functions, use one of `override`, `final`, or `virtual`.
- Use `nullptr` instead of `NULL` or `0`.
- Avoid prefixing class names with "I" (use `Player` instead of `IPlayer`). For concrete implementations, use `PlayerImpl` or a more descriptive name if possible.
- For nested namespaces, use the compact syntax `namespace a::b::c { … }`.