function fig_steps_state_machine()
%FIG_STEPS_STATE_MACHINE  会话状态机：IDLE → 倒计时 → OPEN → CLOSED。
%   只画状态框与迁移箭头，图内不放任何说明性文字。

    S = fig_style();
    D = load_data(S, 'steps_state_machine');
    H = geom_helpers();
    fig = figure('Color', 'w', 'Visible', 'off', 'Position', [100 100 900 240]);
    ax = axes(fig, 'Position', [0.02 0.10 0.96 0.80], ...
              'XLim', [0 100], 'YLim', [0 100], 'Visible', 'off');
    hold(ax, 'on');

    n = numel(D.labels);
    w = 21;
    gap = 4.0;
    x0 = (100 - (n * w + (n - 1) * gap)) / 2;
    y0 = 25;
    h = 50;
    for i = 1:n
        x = x0 + (i - 1) * (w + gap);
        color = pick_color(S, D.edges{i});
        rectangle(ax, 'Position', [x, y0, w, h], 'FaceColor', 'w', ...
                  'EdgeColor', color, 'LineWidth', 1.2);
        text(ax, x + w / 2, y0 + h / 2, D.labels{i}, ...
             'HorizontalAlignment', 'center', 'VerticalAlignment', 'middle', ...
             'Interpreter', 'none');
        if i < n
            H.arrow(ax, [x + w + 0.5, y0 + h / 2], ...
                    [x + w + gap - 0.5, y0 + h / 2], color);
        end
    end
    hold(ax, 'off');
    fig_save(fig, S, 'steps-state-machine.png');
end