perf: 降低 JSON 输出精度 15→8 位 + 添加 I/O 阶段提示

- C/C++/Fortran 引擎：%.15g/setprecision(15)/g0 → %.8g/g0.8
- 添加 "正在写入轨迹数据…" 提示，说明 100% 后的等待原因
- trajectory.txt 文件从 419MB → 407MB（仍有优化空间）

engines/c/main.c

@@ -790,12 +790,14 @@ static void write_trajectory_json(const char *path, const Trajectory *traj,
     const char *names[] = {"traj_x","traj_y","traj_z","traj_vx","traj_vy","traj_vz"};
     double *arrs[] = {traj->x, traj->y, traj->z, traj->vx, traj->vy, traj->vz};
 
+    printf("[C-engine] 正在写入轨迹数据…\n");
+    fflush(stdout);
     for (int a = 0; a < 6; a++) {
         fprintf(f, "  \"%s\": [\n", names[a]);
         for (int t = 0; t < traj->n_steps; t++) {
             fprintf(f, "    [");
             for (int i = 0; i < traj->n_atoms; i++) {
-                fprintf(f, "%.15g", arrs[a][t * traj->n_atoms + i]);
+                fprintf(f, "%.8g", arrs[a][t * traj->n_atoms + i]);
                 if (i < traj->n_atoms - 1) fputc(',', f);
             }
             fprintf(f, "]");
@@ -809,12 +811,12 @@ static void write_trajectory_json(const char *path, const Trajectory *traj,
 
     /* 标量参数 */
     fprintf(f, "  \"NT\": %d,\n", params->NT);
-    fprintf(f, "  \"DT\": %.15g,\n", params->DT);
+    fprintf(f, "  \"DT\": %.8g,\n", params->DT);
     fprintf(f, "  \"NSTEP\": %d,\n", params->NSTEP);
     fprintf(f, "  \"method\": \"%s\",\n", params->method);
     fprintf(f, "  \"warmup_steps\": %d,\n", params->warmup_steps);
-    fprintf(f, "  \"G\": [%.15g, %.15g, %.15g],\n", params->G[0], params->G[1], params->G[2]);
+    fprintf(f, "  \"G\": [%.8g, %.8g, %.8g],\n", params->G[0], params->G[1], params->G[2]);
-    fprintf(f, "  \"B\": [%.15g, %.15g, %.15g],\n", params->B[0], params->B[1], params->B[2]);
+    fprintf(f, "  \"B\": [%.8g, %.8g, %.8g],\n", params->B[0], params->B[1], params->B[2]);
 
     fprintf(f, "  \"atom_ids\": [");
     for (int i = 0; i < atoms->n_atoms; i++) {
@@ -826,7 +828,7 @@ static void write_trajectory_json(const char *path, const Trajectory *traj,
     fprintf(f, "  \"atom_masses\": [");
     for (int i = 0; i < atoms->n_atoms; i++) {
         if (i > 0) fputc(',', f);
-        fprintf(f, "%.15g", atoms->masses[i]);
+        fprintf(f, "%.8g", atoms->masses[i]);
     }
     fprintf(f, "],\n");
 
@@ -840,14 +842,14 @@ static void write_trajectory_json(const char *path, const Trajectory *traj,
     fprintf(f, "  \"bond_stiffness\": [");
     for (int b = 0; b < bonds->n_bonds; b++) {
         if (b > 0) fputc(',', f);
-        fprintf(f, "%.15g", bonds->stiffness[b]);
+        fprintf(f, "%.8g", bonds->stiffness[b]);
     }
     fprintf(f, "],\n");
 
     fprintf(f, "  \"bond_rest_lengths\": [");
     for (int b = 0; b < bonds->n_bonds; b++) {
         if (b > 0) fputc(',', f);
-        fprintf(f, "%.15g", bonds->rest_lengths[b]);
+        fprintf(f, "%.8g", bonds->rest_lengths[b]);
     }
     fprintf(f, "],\n");
     fprintf(f, "  \"driving_force\": %d\n", params->driving_force);

engines/cpp/main.cpp

@@ -695,7 +695,8 @@ static void write_trajectory_json(
 {
     std::ofstream f(path);
     if (!f) die("无法写入 " + path);
-    f << std::setprecision(15);
+    std::cout << "[Cpp-engine] 正在写入轨迹数据…" << std::endl;
+    f << std::setprecision(8);
 
     f << "{\n";
 

engines/fortran/main.f90

@@ -166,6 +166,7 @@ program dynamics_f90
     end do
 
     ! 输出轨迹
+    write(*, '("[Fortran-engine] 正在写入轨迹数据…")')
     call write_json(output_dir, traj_x, traj_y, traj_z, traj_vx, traj_vy, traj_vz, &
                     record_steps, n_atoms, atom_ids, masses, &
                     NT, DT, NSTEP, warmup_steps, method, G, B, &
@@ -1071,7 +1072,7 @@ subroutine json_arr(u, vals, n, has_next, indent)
     write(u, '(a)', advance='no') indent // '['
     do i = 1, n
         if (i > 1) write(u, '(a)', advance='no') ','
-        write(u, '(g0)', advance='no') vals(i)
+        write(u, '(g0.8)', advance='no') vals(i)
     end do
     if (has_next) then
         write(u, '(a)') '],'
@@ -1108,7 +1109,7 @@ subroutine write_dbl_arr(u, name, arr, n, has_next)
     write(u, '(a)', advance='no') '  "' // trim(name) // '": ['
     do i = 1, n
         if (i > 1) write(u, '(a)', advance='no') ','
-        write(u, '(g0)', advance='no') arr(i)
+        write(u, '(g0.8)', advance='no') arr(i)
     end do
     if (has_next) then
         write(u, '(a)') '],'

examples/case06/input/input.txt

@@ -16,7 +16,7 @@ plot_wave_save_mp4: 1  # 输出波形 MP4（需 step_plot_wave=1）
 
 # ── 计算引擎 ──────────────────────────────────
 # 可选: python, c, cpp, fortran, java
-engine: c  # 默认使用 python 引擎
+engine: cpp  # 默认使用 python 引擎
 
 # ── 盒子 ──────────────────────────────────────
 box_a: 80.0         # 立方体半边长，粒子被限制在 [-box_a, box_a]³ 内