109 plain event from handler flow

// PINS A BUG (red until fixed): calling a plain (no-effect-branch) event from
// inside a handler-bearing flow emits a 2-arg handler call to a 1-arg handler.
//
// `plain` has no effect branches, so its handler is `handler(input)` (1 arg).
// But the emitter threads the flow's Handlers struct into EVERY event call in
// the flow, producing `plain_event.handler(.{ .v = r }, Handlers_0)` — 2 args —
// which the Zig backend rejects: "expected 1 argument(s), found 2".
//
// This is the canonical vaxis shape: `! key k |> quit()` calls a plain event
// from inside an effect-handler flow. So this bug blocks the vaxis pattern.
const std = @import("std");

~pub event emit-one { }
! step usize
| done usize

~proc emit-one|zig {
    step(7);
    return .{ .done = 35 };
}

~pub event plain { v: usize }

~proc plain|zig {
    std.debug.print("plain = {d}\n", .{v});
}

~emit-one()
! step _ |> _
| done r |> plain(v: r)