098 capture with effect branch event

// Test: `~capture` integrates with effect-branch events.
//
// Surfaced 2026-05-25 designing the effect-branches throughput benchmark.
// The user-facing intent: producer fires `! v` for each yielded value;
// consumer wraps the iteration in `~capture` to accumulate. Mirrors the
// canonical `~for` + capture pattern from how-capture-works (the blog),
// but with a user-defined effect-branch event in the iteration slot.
//
// Current behavior (the bug): the transform that lowers `~capture` to
// inline Zig calls the inner event's handler without passing the
// consumer's comptime handler struct (`__H`). Effect-branch events have
// signature `handler(Input, comptime __H: type)`, so the call fails with
// Zig's `expected 2 argument(s), found 1`.
//
// Expected (when fixed): proc yields 0, 1, 2 via `! v`. Capture
// accumulates them into `total`. Output is `total = 3`.

~import std/io

const std = @import("std");

~pub event yields { n: u64 }
! v u64

~proc yields|zig {
    for (0..n) |i| v(i);
}

~capture(expr: { total: @as(u64, 0) })
| as c |> yields(n: 3)
    ! v x => captured { total: c.total + x }
| captured final |> std/io:print.ln("total = {{ final.total:d }}")