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}