With my M1 MBA, I am still on macOS 15. To compile it, just remove the two lines with

  opts.languageVersion = .version4_0

or surround them with

  if #available(macOS 26.0, *) {
    opts.languageVersion = .version4_0
  }

You'll miss out on a prefill speedup of 2.4x (as it yields 11.24x faster attention), according to the git comments, but it works. (On the 8-GPU-core MBA M1, I get 5-6 tok/s.)