refactor: binary operators of `quantity` and `quantity_point` are now hidden friends #8

Job	Run time
std::format none C++23 GCC-14 Release	1m 41s
std::format none C++23 GCC-14 Debug	1m 52s
std::format none C++23 Clang-18 Release	2m 39s
fmtlib none C++23 GCC-14 Release	2m 9s
fmtlib none C++23 GCC-14 Debug	1m 47s
fmtlib none C++23 Clang-18 Release	2m 38s
	12m 46s

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