refactor: binary operators of `quantity` and `quantity_point` are now hidden friends #32

Job	Run time
std::format none C++23 GCC-14 Release	2m 5s
std::format none C++23 GCC-14 Debug	2m 10s
std::format none C++23 Clang-18 Release	2m 48s
fmtlib none C++23 GCC-14 Release	2m 5s
fmtlib none C++23 GCC-14 Debug	1m 56s
fmtlib none C++23 Clang-18 Release	3m 0s
	14m 4s

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