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

compute.py

@@ -697,8 +697,6 @@ def run_from_config(config, out_dir=None):
     global GRAVITY_FIELD, GRAVITY_INTERACTION, ELASTIC_FORCE, DAMPING_FORCE, GRAVITY_STRENGTH
     global DRIVING_FORCE, DRIVER_DATA
 
-    global use_marker
-    ...
     box_a = float(config.get("box_a", 10.0))
     raw_alpha = config.get("alpha", 0.2)
     if isinstance(raw_alpha, (list, tuple)):

dynamics.py

@@ -238,6 +238,7 @@ 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)
+                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
@@ -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:

examples/case06/input/bond.txt

@@ -1,2 +1,2 @@
 bond_name k rest_length
-k1 1.0 1.0
+k1 10.0 1.0

examples/case06/input/input.txt

@@ -72,7 +72,7 @@ T_total:  100.0
 NSTEP: 100
 
 # ── 时间步长 ──────────────────────────────────
-DT:    0.01         # 时间步长 (s)
+DT:    0.001         # 时间步长 (s)
 
 # 抽帧范围：只保存 [sample_start, sample_end) 区间内的帧
 sample_start: null        # null 表示从头开始（帧索引从 0 起）