✨ feat: 灵台引擎 Python 绑定与消息路径接入（WP-15/WP-03 交界）

- engine/bindings：nanobind 官方薄绑定包 hunjing-lingtai（LingtaiEngine RAII、HjStatus→LingtaiError 映射、HJ_ABI_VERSION 导出）；scikit-build-core 构建，直接编译引擎源码自包含
- backend 以 editable 路径依赖接入；typings/lingtai.pyi 类型存根（mypy strict 全绿）
- app/core/lingtai.py：懒初始化单例 + LingtaiUnavailable fail-closed（禁止宿主第二实现降级）
- 迁移 3：message.parser_version；post_message 的 markdown 输入经 hj_transform 规范化为 canonical blocks（展开 code/mermaid/math 并标注 parser_version，契约第 4 节）
- 测试 29 项全绿：消息展开、Python/C++ golden 字节一致、引擎注入失败时消息路径显式 500（fail-closed 验证）

backend/pyproject.toml

@@ -13,6 +13,7 @@ dependencies = [
     "asyncpg>=0.30",
     "uuid-utils>=0.10",
     "greenlet>=3.5.5",
+    "hunjing-lingtai",
 ]
 
 [dependency-groups]
@@ -45,7 +46,11 @@ select = ["E", "F", "I", "UP", "B", "ASYNC"]
 [tool.mypy]
 python_version = "3.12"
 strict = true
+mypy_path = "typings"
 
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
 testpaths = ["tests"]
+
+[tool.uv.sources]
+hunjing-lingtai = { path = "../engine/bindings", editable = true }