test: 新增 codegen 层单元测试 (7 tests)

- test_codegen.c: 构造 AST → codegen_module → LLVMVerifyModule
- 覆盖: 函数+返回字面量, if/else 分支, 二元运算(1+2*3)
- 总单元测试: 41+15+9+7 = 72
- 基于 Codex 分析报告 §5-4 技术债务补偿

CMakeLists.txt

@@ -92,3 +92,13 @@ target_include_directories(l_lang_sema_test PRIVATE
     src/util src/lexer src/ast src/parser src/sema src/codegen src/driver
     test
 )
+
+# Codegen 测试
+add_executable(l_lang_codegen_test test/test_codegen.c)
+target_link_libraries(l_lang_codegen_test PRIVATE l_lang_lib)
+target_include_directories(l_lang_codegen_test PRIVATE
+    ${CMAKE_SOURCE_DIR}/include
+    ${LLVM_INCLUDE_DIRS}
+    src/util src/lexer src/ast src/parser src/sema src/codegen src/driver
+    test
+)