016 subflow bind then construct

// Test 350_016: bare-return head bind, then construct (`: v => branch v`).
//
// A subflow whose head is a bare-return (`-> T`) event binds the produced
// value with `:` and immediately constructs the outer event's branch from it:
//   ~proc1 = mk(x: n): v => ok v
// The `=> ok v` construct must survive parsing as a void-branch continuation
// (it has no `| branch binding` wrapper — the `: v` bind moved to the head).
// Previously the orphaned `=>` was dropped and the handler never returned.

const std = @import("std");
~import std/io

~event mk { x: i32 } -> i32
~proc mk|zig { return x * 2; }

~event proc1 { n: i32 }
| ok i32

~proc1 = mk(x: n): v => ok v

~proc1(n: 21)
| ok r |> std/io:print.ln("result: {{ r:d }}")