[:0]u8 aliases string (fat) and params already ABI-thin to char*, but
a foreign -> [:0]u8 return silently resolves to plain u8, and ?[:0]u8
never resolves at all (LLVM emission panic) even though ?string works.
Design contract recorded: ?[:0]u8 lowers to a nullable char* at the
boundary, length synthesized on the sx side; until then such returns
must be diagnosed, not mis-typed.
