refactor: 源码迁至 engines/src/，删除旧的 engines/{c,cpp,fortran}/

- 删除 engines/{c,cpp,fortran}/ 目录（源码和 Makefile 已移至 src/）
- engines/src/{c,cpp,fortran}/: 清理 main.*/bak 等无用文件
- Makefile 改为直接编译 DLL 到 engines/release/
- .gitignore: 更新路径指向 engines/src/*/build/
- engine_dll.py: 更新注释中的编译命令路径

engines/src/fortran/Makefile

@@ -1,47 +1,44 @@
-# engines/fortran/Makefile
+# engines/src/fortran/Makefile
+# 编译 DLL 到 engines/release/（主程序通过 ctypes 直接调用）
 
 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
+  STATIC_FLAGS = -static
 else
   STATIC_FLAGS =
 endif
 
-TARGET = build/dynamics_f90.exe
+# DLL 输出到 engines/release/
+DLL_DIR = ../../release
 
 ifeq ($(UNAME_S),Linux)
-  DLL_TARGET = build/dynamics_f90.so
+  DLL_TARGET = $(DLL_DIR)/dynamics_f90.so
   DLL_FLAGS  = -shared -fPIC
 else ifeq ($(UNAME_S),Darwin)
-  DLL_TARGET = build/dynamics_f90.dylib
+  DLL_TARGET = $(DLL_DIR)/dynamics_f90.dylib
   DLL_FLAGS  = -dynamiclib
 else
-  DLL_TARGET = build/dynamics_f90.dll
+  DLL_TARGET = $(DLL_DIR)/dynamics_f90.dll
   DLL_FLAGS  = -shared -fPIC
 endif
 
 .PHONY: all dll clean
 
-all: $(TARGET)
+all: dll
 
 dll: $(DLL_TARGET)
 
-$(TARGET): $(SRCS) | build
-	$(FC) $(FFLAGS) $(STATIC_FLAGS) -o $@ $(SRCS)
-	@echo "  === Fortran engine built: $@ ==="
-
-$(DLL_TARGET): $(LIB_SRC) | build
+$(DLL_TARGET): $(LIB_SRC) | $(DLL_DIR)
 	$(FC) $(FFLAGS) $(STATIC_FLAGS) $(DLL_FLAGS) -o $@ $(LIB_SRC)
 	@echo "  === Fortran DLL built: $@ ==="
 
-build:
-	mkdir -p build
+$(DLL_DIR):
+	mkdir -p $(DLL_DIR)
 
 clean:
-	rm -rf build *.o *.mod
+	rm -f $(DLL_TARGET)