Fix deprecated float suffix and migrate to C++17 for compatibility

[0x1306a94]

This pull request addresses two issues in the current codebase:

1. Deprecated floating-point literal suffix:
   The code uses literals like 255.0d, which are not valid in modern C++ standards. The suffix d is not standard C++ and causes build errors on some compilers. These have been replaced with standard double literals such as 255.0.

2. Nested namespace definition syntax:
   The code uses nested namespace definitions (namespace a::b {}), which require at least C++17. To ensure consistent support across modern compilers, this PR updates the required C++ standard to C++17.