For what it’s worth there’s a reason you’re supposed to do this kind of access through memcpy, not by dereferencing made up pointers.
> There exists an instruction to do that load in Wasm, there's a builtin to check that 12345 points to addressable memory, the load is valid at the assembly level, the standard says the implementation should define this to be consistent with the addressing structure of the execution environment, why the heck are we playing games and allowing the compiler to say, "nope, that's not valid, so your entire program is invalid, and we can do what ever we want, no diagnostic required"?
Because the language standard is defined to target a virtual machine as output, not any given implementation. That virtual machine is then implemented on various platforms, but the capabilities of the underlying system aren’t directly accessible - they are only there to implement the C virtual machine. That’s why C can target so many different target machines.
