✨ 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）

engine/bindings/src/lingtai.cpp

@@ -0,0 +1,88 @@
+// 灵台引擎官方 Python 绑定（nanobind，ADR-016）。
+// 薄封装：类型转换、句柄 RAII 与错误映射；不复制业务语义（契约 lingtai-ffi 第 8 节）。
+#include <stdexcept>
+#include <string>
+
+#include <nanobind/nanobind.h>
+#include <nanobind/stl/string.h>
+
+#include "hunjing/lingtai.h"
+
+namespace nb = nanobind;
+
+namespace {
+
+struct LingtaiError : std::runtime_error {
+  using std::runtime_error::runtime_error;
+};
+
+struct PyEngine {
+  HjEngine *handle = nullptr;
+
+  PyEngine() {
+    const HjStatus st = hj_engine_new(HJ_ABI_VERSION, &handle);
+    if (st != HJ_OK) {
+      throw LingtaiError(std::string("灵台引擎初始化失败：") + hj_last_error());
+    }
+  }
+
+  PyEngine(const PyEngine &) = delete;
+  PyEngine &operator=(const PyEngine &) = delete;
+
+  ~PyEngine() {
+    if (handle != nullptr) {
+      hj_engine_free(handle);
+      handle = nullptr;
+    }
+  }
+};
+
+const char *status_name(HjStatus st) {
+  switch (st) {
+    case HJ_OK: return "HJ_OK";
+    case HJ_ERR_INVALID_ARGUMENT: return "HJ_ERR_INVALID_ARGUMENT";
+    case HJ_ERR_OUT_OF_MEMORY: return "HJ_ERR_OUT_OF_MEMORY";
+    case HJ_ERR_UNSUPPORTED_ABI: return "HJ_ERR_UNSUPPORTED_ABI";
+    case HJ_ERR_HANDLE_INVALID: return "HJ_ERR_HANDLE_INVALID";
+    case HJ_ERR_REENTRANCY: return "HJ_ERR_REENTRANCY";
+    case HJ_ERR_PARSE_FAILED: return "HJ_ERR_PARSE_FAILED";
+    case HJ_ERR_LIMIT_EXCEEDED: return "HJ_ERR_LIMIT_EXCEEDED";
+    case HJ_ERR_POLICY_DENIED: return "HJ_ERR_POLICY_DENIED";
+    case HJ_ERR_SIGNATURE_INVALID: return "HJ_ERR_SIGNATURE_INVALID";
+    case HJ_ERR_INDEX_CORRUPT: return "HJ_ERR_INDEX_CORRUPT";
+    case HJ_ERR_BUFFER_TOO_SMALL: return "HJ_ERR_BUFFER_TOO_SMALL";
+    case HJ_ERR_INTERNAL: return "HJ_ERR_INTERNAL";
+  }
+  return "HJ_ERR_UNKNOWN";
+}
+
+}  // namespace
+
+NB_MODULE(lingtai, m) {
+  m.doc() = "灵台引擎（Lingtai Engine）官方 Python 绑定";
+
+  m.attr("HJ_ABI_VERSION") = HJ_ABI_VERSION;
+  m.def("abi_version", [] { return hj_abi_version(); }, "引擎构建时的 ABI major");
+  m.def("transform_version", [] { return std::string(hj_transform_version()); },
+        "Markdown 解析器版本");
+
+  nb::exception<LingtaiError>(m, "LingtaiError");
+
+  nb::class_<PyEngine>(m, "LingtaiEngine")
+      .def(nb::init<>())
+      .def(
+          "transform_markdown",
+          [](PyEngine &self, const std::string &input) -> std::string {
+            HjBuffer *buf = nullptr;
+            const HjStatus st =
+                hj_transform_markdown(self.handle, input.data(), input.size(), &buf);
+            if (st != HJ_OK) {
+              throw LingtaiError(std::string(status_name(st)) + ": " + hj_last_error());
+            }
+            std::string out(static_cast<const char *>(hj_buffer_data(buf)), hj_buffer_len(buf));
+            hj_buffer_free(buf);
+            return out;
+          },
+          nb::arg("input"),
+          "Markdown → canonical ContentBlock JSON（确定性、键序固定）");
+}