fix: save_trajectory=0 时删除 trajectory.txt，所有引擎保持一致

Python 引擎：run_simulation 已正确支持 save_trajectory ✅
外部引擎（C/C++/Fortran）：save_trajectory_txt 仅在
  save_trajectory=1 时调用；display.txt 生成后删除 trajectory.txt
补充：移除 compute.py 中重复的 'global use_marker' 和占位符

dynamics.py

@@ -238,7 +238,8 @@ def run_case(config_path, runtime_base, input_dir="input", output_dir="output",
                 # 外部引擎写完整 trajectory.txt，后续抽帧
                 traj_x, traj_y, traj_z, traj_vx, traj_vy, traj_vz = compute.run_engine(
                     engine, input_dir_abs, output_dir_abs, config)
-                compute.save_trajectory_txt(traj_x, traj_y, traj_z, traj_vx, traj_vy, traj_vz, str(runtime_base))
+                if int(config.get("save_trajectory", 0)):
+                    compute.save_trajectory_txt(traj_x, traj_y, traj_z, traj_vx, traj_vy, traj_vz, str(runtime_base))
 
             _elapsed = _time.time() - _t0
             print(f"[run] 引擎: {engine}  计算完成: {record_steps} 步  {_elapsed:.3f} s")
@@ -309,6 +310,16 @@ def run_case(config_path, runtime_base, input_dir="input", output_dir="output",
                 header_fields=hf)
             print(f"[run] 从 trajectory.txt 抽帧生成 display.txt ({n_frames} 帧)")
 
+        # 外部引擎：save_trajectory=0 时清理 trajectory.txt
+        save_traj = int(config.get("save_trajectory", 0))
+        if engine != "python" and not save_traj:
+            traj_path = os.path.join(output_dir_abs, "trajectory.txt")
+            try:
+                os.remove(traj_path)
+                print(f"[run] save_trajectory=0，已删除 {traj_path}")
+            except OSError:
+                pass
+
         # 4. 绘图（可选）
         if not no_plot and config.get("step_plot", 1):
             try: