refactor: 移除旧子进程模式的 main.* 源文件，Makefile 只编译 DLL

- 删除 engines/c/main.c, engines/cpp/main.cpp, engines/fortran/main.f90
- Makefile 移除 all/exe 构建目标，默认只编译 DLL

engines/cpp/Makefile

@@ -1,7 +1,7 @@
 # engines/cpp/Makefile
+# 编译 DLL（主程序通过 ctypes 直接调用）
 
 CXX     = g++
-SRCS    = main.cpp
 LIB_SRC = dynamics_lib.cpp
 
 UNAME_S := $(shell uname -s 2>/dev/null || echo Windows)
@@ -15,8 +15,6 @@ else
   STATIC_FLAGS =
 endif
 
-TARGET = build/dynamics_cpp.exe
-
 ifeq ($(UNAME_S),Linux)
   DLL_TARGET = build/dynamics_cpp.so
   DLL_FLAGS  = -shared -fPIC
@@ -30,14 +28,10 @@ endif
 
 .PHONY: all dll clean
 
-all: $(TARGET)
+all: dll
 
 dll: $(DLL_TARGET)
 
-$(TARGET): $(SRCS) | build
-	$(CXX) $(CXXFLAGS) $(STATIC_FLAGS) -o $@ $(SRCS)
-	@echo "  === C++ engine built: $@ ==="
-
 $(DLL_TARGET): $(LIB_SRC) | build
 	$(CXX) $(CXXFLAGS) $(STATIC_FLAGS) $(DLL_FLAGS) -o $@ $(LIB_SRC)
 	@echo "  === C++ DLL built: $@ ==="