186 param default on proc path

// CONTROL for 400_185 — the same default, same call, PROC impl instead of a
// subflow. Green before the default was a real concept and green after, which
// is what makes 400_185 attributable to the emission path rather than to
// defaults being unimplemented in general.
//
// Hold this one: if a proper default implementation ever regresses it, the
// accident it replaced was load-bearing and that is worth knowing.
~import std/io

~tor add { a: i32, b: i32 = 5 } -> i32

~proc add|zig {
    return a + b;
}

~add(a: 1): r |> std/io:print.ln("{{ r:d }}")