refactor: binary operators of `quantity` and `quantity_point` are now hidden friends #92

Job	Run time
std::format none C++23 GCC-14 Release	1m 46s
std::format none C++23 GCC-14 Debug	1m 53s
std::format none C++23 Clang-18 Release	43s
fmtlib none C++23 GCC-14 Release	1m 45s
fmtlib none C++23 GCC-14 Debug	1m 53s
fmtlib none C++23 Clang-18 Release	51s
	8m 51s

Provide feedback