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/src/fortran/Makefile

@@ -0,0 +1,47 @@
+# engines/fortran/Makefile
+
+FC      = gfortran
+FFLAGS  = -O3 -march=native -Wall -Wextra
+SRCS    = main.f90
+LIB_SRC = dynamics_lib.f90
+
+UNAME_S := $(shell uname -s 2>/dev/null || echo Windows)
+
+ifeq ($(UNAME_S),Windows)
+  STATIC_FLAGS = -static-libgcc -static-libgfortran -static-libquadmath
+else
+  STATIC_FLAGS =
+endif
+
+TARGET = build/dynamics_f90.exe
+
+ifeq ($(UNAME_S),Linux)
+  DLL_TARGET = build/dynamics_f90.so
+  DLL_FLAGS  = -shared -fPIC
+else ifeq ($(UNAME_S),Darwin)
+  DLL_TARGET = build/dynamics_f90.dylib
+  DLL_FLAGS  = -dynamiclib
+else
+  DLL_TARGET = build/dynamics_f90.dll
+  DLL_FLAGS  = -shared -fPIC
+endif
+
+.PHONY: all dll clean
+
+all: $(TARGET)
+
+dll: $(DLL_TARGET)
+
+$(TARGET): $(SRCS) | build
+	$(FC) $(FFLAGS) $(STATIC_FLAGS) -o $@ $(SRCS)
+	@echo "  === Fortran engine built: $@ ==="
+
+$(DLL_TARGET): $(LIB_SRC) | build
+	$(FC) $(FFLAGS) $(STATIC_FLAGS) $(DLL_FLAGS) -o $@ $(LIB_SRC)
+	@echo "  === Fortran DLL built: $@ ==="
+
+build:
+	mkdir -p build
+
+clean:
+	rm -rf build *.o *.mod