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/engine_dll.py

@@ -67,7 +67,7 @@ def _dll_candidates(engine: str) -> list[str]:
     ext = _SUFFIX.get(sys, ".so")
     eng_dir = _ENGINE_DIR.get(engine, engine)
     name    = _DLL_NAME.get(engine, f"dynamics_{engine}")
-    base = os.path.join(_HERE, eng_dir, "build", name)
+    base = os.path.join(_HERE, "release", name)
     return [
         base + ext,
         base + ".dll",