031 midchain bind produce in handler

// Mid-chain `: bind` of a BARE-RETURN value inside a BRANCH HANDLER:
//   ~start() | created c0 |> mk(x: c0): c1 |> use-it(x: c1)
// `mk` is `-> i64`; its result is bound `: c1` mid-chain and used downstream.
// This is the metacircular `coordinate` pipeline's shape — it consumes the
// bare-return `frontend(ctx): c1 |> analysis(ctx: c1)` inside the `| created`
// handler. Currently the emitter drops the binding: generated Zig references
// an undeclared `c1` (use of undeclared identifier 'c1').
//
// Distinct from the subflow produce-arm bug (020_029/030, fixed): that was a
// subflow body; this is a `:` bind on a bare-return STEP inside a `|` handler.
~import std/io
const std = @import("std");

~pub event start {}
| created i64

~proc start|zig { return .{ .created = 5 }; }

~pub event mk { x: i64 } -> i64
~mk -> x + 1

~pub event use-it { x: i64 }
~proc use-it|zig { std.debug.print("got {d}\n", .{x}); }

~start()
| created c0 |> mk(x: c0): c1 |> use-it(x: c1)