✨ 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/uv.lock

@@ -347,6 +347,7 @@ dependencies = [
     { name = "asyncpg" },
     { name = "fastapi" },
     { name = "greenlet" },
+    { name = "hunjing-lingtai" },
     { name = "pydantic" },
     { name = "pydantic-settings" },
     { name = "sqlalchemy" },
@@ -369,6 +370,7 @@ requires-dist = [
     { name = "asyncpg", specifier = ">=0.30" },
     { name = "fastapi", specifier = ">=0.115" },
     { name = "greenlet", specifier = ">=3.5.5" },
+    { name = "hunjing-lingtai", editable = "../engine/bindings" },
     { name = "pydantic", specifier = ">=2.9" },
     { name = "pydantic-settings", specifier = ">=2.6" },
     { name = "sqlalchemy", specifier = ">=2.0" },
@@ -385,6 +387,11 @@ dev = [
     { name = "ruff", specifier = ">=0.8" },
 ]
 
+[[package]]
+name = "hunjing-lingtai"
+version = "0.1.0"
+source = { editable = "../engine/bindings" }
+
 [[package]]
 name = "idna"
 version = "3.19"