Discussible requirements:

• Get one integer;
• Get random bytes range;
• POSIX: use read from dev (urandom)/ getrandom(2);
• Windows: Use BCryptGenRandom()/ CryptGenRandom() (for older windows maybe) - https://learn.microsoft.com/en-us/windows/win32/api/bcrypt/nf-bcrypt-bcryptgenrandom - all these funcs requires linking with bcrypt.lib and crates handle (it also may be used for another crypto operations, like HMAC);
• Os/Random.hpp for uniform API.

POSIX also allows to use random asynchronously, possibly can be used in API like "RandomAsync.hpp" .