042 capture if for multi

// Capture-as-events: BUILT 2026-06-11 (was the frontier pin for the
// dissolution; green since the [transform]proc migration). The primitives:
//   - `capture { ... }` and `captured { ... }` are the SAME shape — events
//     taking one SOURCE BLOCK (bare braces, no parens; ratified 2026-06-11 —
//     `name { ... }` is unambiguously an invocation since `=>` took over
//     branch construction, 210_047). `capture { X }` seeds, `captured { X }`
//     updates, and `captured { ... }` is just an event call, so it composes
//     anywhere in a chain.
//   - `! as acc` binds the accumulator as an EFFECT cell — live/during the loop
//     (effects fire during; the cell is mutated each iteration).
//   - `| captured result` is the CONTINUATION — the after-value, handed off once.
//     It is OPTIONAL (`| ?captured`): if the work happened via side effects in
//     the effect-branch, you may decline the final reading (cf. `~for`'s `| ?done`).
//   - The `{ sum: ..., count: ... }` block is OPAQUE — the `captured` handler
//     interprets it via the projector and emits per-target writes into the cell.
//
// Behavior when built: sum/count over 1, 3, 6, 8, 10; values > 5 are 6, 8, 10
// (sum = 24, count = 3, avg = 8).

~import std/io
~import std/control

~capture { sum: @as(i64, 0), count: @as(i32, 0) }
! as acc |> for(&[_]i32{1, 3, 6, 8, 10})
    ! each item |> if(item > 5)
        | then |> captured { sum: acc.sum + @as(i64, item), count: acc.count + 1 }
        | else |> captured { acc.sum, acc.count }
| captured result |> std/io:print.ln("{{ @divTrunc(result.sum, @as(i64, result.count)):d }}")