DropdownMenu

// ── DropdownMenu（同 Menu：trigger+popover）──
pub fn buildDropdownMenu(scope: *ui.Scope, cx: *ui.Cx) anyerror!*ui.Node {
    const a = cx.allocator;
    const c = try col(cx, 8);
    try c.appendChild(a, try label(cx, "Click the trigger to open the dropdown"));
    const items = [_]W.DropdownItem{
        .{ .id = "new", .label_text = "New File" },
        .{ .id = "open", .label_text = "Open…" },
        .{ .id = "sep", .kind = .separator },
        .{ .id = "save", .label_text = "Save" },
    };
    const dm = try W.DropdownMenu(.{ .items = &items }).mount(scope, cx);
    try dm.trigger.appendChild(a, try W.Button(.{ .label = "File ▾", .variant = .secondary }).mount(scope, cx));
    dm.wrapper.meta.ownership.meta.test_id = "story.dropdown.trigger";
    try c.appendChild(a, dm.wrapper);
    return c;
}