✨ feat: 灵台引擎脾神（hj_transform）确定性 Markdown 解析器（WP-15）

- C ABI 新增 hj_transform_version / hj_transform_markdown（契约登记同步）；HjBuffer 真实实现（头部+紧随数据）
- v1 解析（hunjing-markdown-v1）：fenced code/mermaid 提取、独占行 $$ 块级数学、未闭合结构保持源码态、其余归并 markdown block；canonical JSON 键序确定、UTF-8 校验、1MiB/256 block/64KiB 行硬限额
- 引擎内部设施抽取（engine_internal.h：last_error/句柄表/buffer 共享）
- doctest 12 个用例（含 1 万次随机鲁棒性冒烟，固定种子可复现）；libFuzzer 目标（-DHJ_BUILD_FUZZER，Apple clang 无 fuzzer 运行时，正式跑在 Linux CI）
- nm 验证 9 个导出符号全部 hj_*

engine/CMakeLists.txt

@@ -17,14 +17,29 @@ endif()
 set(CMAKE_CXX_VISIBILITY_PRESET hidden)
 set(CMAKE_VISIBILITY_INLINES_HIDDEN ON)
 
-add_library(hunjing SHARED src/lingtai.cpp)
-target_include_directories(hunjing PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/include)
+add_library(hunjing SHARED src/lingtai.cpp src/transform.cpp)
+target_include_directories(hunjing
+  PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/include
+  PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/src)
 
 # 单元测试（doctest，ADR-025）；单头直接引入，不用 FetchContent 拉网络
-add_executable(lingtai_tests tests/test_core.cpp src/lingtai.cpp)
+add_executable(lingtai_tests tests/test_main.cpp tests/test_core.cpp tests/test_transform.cpp
+  src/lingtai.cpp src/transform.cpp)
 target_include_directories(lingtai_tests PRIVATE
   ${CMAKE_CURRENT_SOURCE_DIR}/include
+  ${CMAKE_CURRENT_SOURCE_DIR}/src
   ${CMAKE_CURRENT_SOURCE_DIR}/third_party)
 
 enable_testing()
 add_test(NAME lingtai_tests COMMAND lingtai_tests)
+
+# fuzz 目标（ADR-025：hj_transform 为必测目标；需要 clang libFuzzer）
+option(HJ_BUILD_FUZZER "Build libFuzzer targets" OFF)
+if(HJ_BUILD_FUZZER)
+  add_executable(fuzz_transform fuzz/fuzz_transform.cpp src/lingtai.cpp src/transform.cpp)
+  target_include_directories(fuzz_transform PRIVATE
+    ${CMAKE_CURRENT_SOURCE_DIR}/include
+    ${CMAKE_CURRENT_SOURCE_DIR}/src)
+  target_compile_options(fuzz_transform PRIVATE -fsanitize=fuzzer,address -g)
+  target_link_options(fuzz_transform PRIVATE -fsanitize=fuzzer,address)
+endif()