fix: display.txt 缺少渲染参数导致盒子不透明

外部引擎（C/C++）直接写 display.txt 时只输出基础物理参数
（DT/NSTEP/method 等），缺少 alpha/ball_color/box_color
等渲染参数，draw.py 读取不到 alpha 回退默认 0.2。

修复：
1. param.json 新增渲染参数（alpha/ball_color/box_color/...）
2. C/C++ 引擎 SimParams 新增对应字段 & JSON 读取
3. C/C++ write_display_txt 写入所有渲染参数 header
4. param.json ball_color/box_color 用数组统一存储

engines/cpp/main.cpp

@@ -44,6 +44,14 @@ struct SimParams {
     double gravity_strength = 1.0;
     int driving_force = 0;
     int save_trajectory = 1;
+    double alpha[6] = {0,0,0,0,0,0};
+    double ball_radius = 0.5;
+    double ball_color[3] = {0.9, 0.2, 0.2};
+    double box_color[3] = {0.8, 0.8, 0.85};
+    int use_marker = 0;
+    double camera_distance = 40.0;
+    double camera_elevation = 0;
+    double camera_azimuth = 0;
 };
 
 // ========================================================================
@@ -152,6 +160,21 @@ static void json_read_double3(const std::string &json, const std::string &key, d
     }
 }
 
+/* 读取 JSON 数组到 double[6] */
+static void json_read_double6(const std::string &json, const std::string &key, double out[6]) {
+    auto pos = json.find("\"" + key + "\"");
+    if (pos == std::string::npos) { for (int i=0;i<6;i++) out[i]=0; return; }
+    pos = json.find('[', pos);
+    if (pos == std::string::npos) { for (int i=0;i<6;i++) out[i]=0; return; }
+    pos++;
+    for (int i = 0; i < 6; i++) {
+        while (pos < json.size() && (json[pos]==' '||json[pos]=='\t'||json[pos]=='\n'||json[pos]==','||json[pos]==']')) pos++;
+        char *end;
+        out[i] = std::strtod(json.c_str() + pos, &end);
+        pos = end - json.c_str();
+    }
+}
+
 /* 解析 param.json */
 static SimParams read_params(const std::string &path) {
     std::string buf = read_file(path);
@@ -174,6 +197,14 @@ static SimParams read_params(const std::string &path) {
     // save_trajectory 默认 1（全量），仅在 JSON 中存在该 key 时覆盖
     if (json_has_key(buf, "save_trajectory"))
         p.save_trajectory = json_read_int(buf, "save_trajectory");
+    json_read_double6(buf, "alpha", p.alpha);
+    p.ball_radius         = json_read_double(buf, "ball_radius");
+    json_read_double3(buf, "ball_color", p.ball_color);
+    json_read_double3(buf, "box_color", p.box_color);
+    p.use_marker          = json_read_int(buf, "use_marker");
+    p.camera_distance     = json_read_double(buf, "camera_distance");
+    p.camera_elevation    = json_read_double(buf, "camera_elevation");
+    p.camera_azimuth      = json_read_double(buf, "camera_azimuth");
     return p;
 }
 
@@ -717,7 +748,20 @@ static void write_display_txt(
     f << "dynamic_steps: " << (params.NT - params.warmup_steps) << "\n";
     f << "T_total: " << T_total << "\n";
     f << "box_a: " << params.box_a << "\n";
-    f << "driving_force: " << params.driving_force << "\n\n";
+    f << "driving_force: " << params.driving_force << "\n";
+    f << "alpha: " << params.alpha[0] << "," << params.alpha[1] << "," << params.alpha[2] << ","
+                   << params.alpha[3] << "," << params.alpha[4] << "," << params.alpha[5] << "\n";
+    f << "ball_radius: " << params.ball_radius << "\n";
+    f << "ball_color_r: " << params.ball_color[0] << "\n";
+    f << "ball_color_g: " << params.ball_color[1] << "\n";
+    f << "ball_color_b: " << params.ball_color[2] << "\n";
+    f << "box_color_r: " << params.box_color[0] << "\n";
+    f << "box_color_g: " << params.box_color[1] << "\n";
+    f << "box_color_b: " << params.box_color[2] << "\n";
+    f << "use_marker: " << params.use_marker << "\n";
+    f << "camera_distance: " << params.camera_distance << "\n";
+    f << "camera_elevation: " << params.camera_elevation << "\n";
+    f << "camera_azimuth: " << params.camera_azimuth << "\n\n";
 
     f << std::fixed << std::setprecision(6);
     for (int t = 0; t < n_steps; t++) {