085 effect branch resume arithmetic

// Test: effect-branch handler produces an arithmetic resume value with `->`.
// The effect is declared `! double i32 -> i32`, so the arm produces with
// `-> n * 2` — the Zig expression evaluates and is sent back as the resume
// value. `->` is the produce glyph (not `=>`: there are no branches).

~import std/io

~pub event request { payload: i32 }
! double i32 -> i32
| done i32

~proc request|zig {
    const result = double(payload);
    return .{ .done = result };
}

~request(payload: 21)
! double n -> n * 2
| done r |> std/io:print.blk {
    result is {{ r:d }}
}