fix: display.txt 缺失 alpha 透明度参数，draw.py 读取不到

alpha（盒子透明度）未写入 display.txt header，
draw.py 回退到默认 0.2 而非 input.txt 配置值。
现在 alpha 通过 header 字段正确传递。

Python 引擎：alpha 支持单值或数组 → 逗号分隔字符串
外部引擎：从 trajectory.txt 读取 alpha 值

compute.py

@@ -1459,7 +1459,8 @@ def run_simulation(save_trajectory=0):
                        "damping_force": str(DAMPING_FORCE),
                        "gravity_strength": str(GRAVITY_STRENGTH),
                        "driving_force": str(DRIVING_FORCE),
-                       "use_marker": str(use_marker)}
+                       "use_marker": str(use_marker),
+                       "alpha": ",".join(str(a) for a in (alpha if isinstance(alpha, list) else [alpha]))}
     )
     print(f"[compute] display.txt 已保存至: {disp_path} ({n_frames_actual} 帧)")