129 effect record payload arm

// A resume arm may carry a RECORD payload: the synthesized union field is an
// inline struct (`result: struct { halved: i32, quadrupled: i64 }`) and the
// handler constructs it with the ordinary branch-ctor field form
// (`=> result { halved: ..., quadrupled: ... }`). The proc consumes it like
// any tagged union — switch, then field access. Scalar-arm sibling: 400_114.
~import std/io

~pub event request { payload: i32 }
! ask i32 | result { halved: i32, quadrupled: i64 } | timeout
| done i64

~proc request|zig {
    const r = ask(payload);
    return switch (r) {
        .result => |v| .{ .done = v.quadrupled + v.halved },
        .timeout => .{ .done = -1 },
    };
}

~request(payload: 20)
! ask n => result { halved: @divTrunc(n, 2), quadrupled: @as(i64, n) * 4 }
| done r |> std/io:print.ln("{{ r:d }}")