> It's been many years since I wrote any Go for a living, but does Go seriously lack a way to say "foo is probably 32 bytes, give me the 32 byte array, and if I'm wrong about how big foo is, let me handle that" ?

Not in the static type signature, but you can do that as a runtime check either by casting and handling the potential panic (as described above) or by checking the size and returning an error if it's not as expected, which is what the library does.