157 pun effect payload bind

// Effect-payload scalar-bind pun (the gap). An effect branch `! emit text`
// binds a scalar payload named `text`; a later stage's same-named param should
// fill FROM that binding with no explicit `text: text` — exactly like the
// return-binding pun in 210_154. IMPLICITLY, it does NOT. Target: `5:punned!`.
//
// Currently RED: `error[KORU100]: unused binding 'text'`.
//
// Root cause (grounded 2026-07-22): NOT "effect payloads aren't pun sources" —
// they are collected (`punContinuationBinding` step 2 adds `cont.binding`). It's
// an ORDERING gap. The AST shapes differ:
//   return-bind  `get-msg(): text |> render-at(x:5)`  ->  producer and consumer
//                are SEPARATE continuations (consumer is a CHILD); the return
//                bind enters scope, THEN the child fills. Works.
//   effect-payload `! emit text |> render-at(x:5)`    ->  the branch payload
//                binding AND the consumer node sit on the SAME continuation
//                (`| emit text => INVOKE render-at`). Step 1 fills cont.node
//                BEFORE step 2 adds `cont.binding` — so `text` isn't in scope
//                when its own consumer is filled. (The step-1-before-step-2
//                order is CORRECT for return binds, where cont.node is the
//                PRODUCER; it's wrong for branch payloads, where cont.node is
//                the CONSUMER and the payload came from upstream.)
//
// Two spellings already WORK today, proving the binding is a live pun source —
// only the fully-implicit form misses: `render-at(x:5, text)` (bare arg) runs;
// `render-at(x:5, text: text)` is REJECTED as a redundant label ("'text' already
// puns to 'text'"). So the compiler advertises the pun, then fails to apply it
// implicitly. Fix: seed the branch payload `cont.binding` into scope BEFORE the
// step-1 fill; keep return_binding/destructure added AFTER.
//
// Found 2026-07-22 building koru-libs/vaxis: `! draw win |> app()` (win the
// draw-effect payload) hit this. `app(win)` works today; this makes `app()` do
// too. See vaxis-component-baton.
const std = @import("std");

~tor render-at { x: i64, text: string }
~proc render-at|zig { std.debug.print("{d}:{s}\n", .{ x, text }); }

~tor source {}
! emit string
~proc source|zig { emit("punned!"); }

~source()
! emit text |> render-at(x: 5)