refactor: 引擎源码移至 src/，编译产物统一至 release/

- 源码: engines/c/ → engines/src/c/
- 源码: engines/cpp/ → engines/src/cpp/
- 源码: engines/fortran/ → engines/src/fortran/
- 编译产物: engines/release/ (静态编译)
  - dynamics_c.exe   (516KB, C 引擎)
  - dynamics_cpp.exe (3.3MB, C++ 引擎)
  - dynamics_f90.exe (988KB, Fortran 引擎)
- compute.py: engine_map 指向 release/
- compute.py: param.json 和校准缓存移至 release/
- engine_dll.py: DLL 搜索路径指向 release/
- release/ 目录纳入 git 版本管理，用户克隆后直接可用

engines/release/_calib_c.json

@@ -0,0 +1 @@
+{"n_atoms": 120, "nt": 1000000, "step_time": 0.0009682028412818909}

engines/release/c.json

@@ -0,0 +1,51 @@
+{
+  "box_a": 300.0,
+  "NT": 1000000,
+  "DT": 0.001,
+  "NSTEP": 500,
+  "warmup_steps": 0,
+  "method": "leapfrog",
+  "G": [
+    0.0,
+    0.0,
+    0.0
+  ],
+  "B": [
+    0.01,
+    0.0,
+    0.01
+  ],
+  "gravity_field": 0,
+  "gravity_interaction": 0,
+  "elastic_force": 1,
+  "damping_force": 1,
+  "gravity_strength": 1.0,
+  "driving_force": 1,
+  "save_trajectory": 0,
+  "alpha": [
+    0.0,
+    0.0,
+    0.0,
+    0.0,
+    0.0,
+    0.0
+  ],
+  "ball_radius": 0.5,
+  "ball_color": [
+    0.2,
+    0.6,
+    0.9
+  ],
+  "box_color": [
+    0.8,
+    0.8,
+    0.85
+  ],
+  "use_marker": 1,
+  "camera_distance": 120.0,
+  "camera_elevation": 0.0,
+  "camera_azimuth": 0.0,
+  "camera_center_x": 60.0,
+  "camera_center_y": 0.0,
+  "camera_center_z": 0.0
+}