✨ 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/tests/test_conversations.py

@@ -2,6 +2,7 @@
 
 import json
 
+import pytest
 from httpx import AsyncClient
 from sqlalchemy import select
 
@@ -146,3 +147,52 @@ class TestAtomicity:
             outbox = (await session.scalars(select(models.OutboxEvent))).all()
             assert [e for e in outbox if e.aggregate_type == "message"] == []
             assert (await session.scalars(select(models.IdempotencyRecord))).all() == []
+
+
+@pytest.mark.usefixtures("client")
+class TestLingtaiNormalization:
+    async def test_markdown_input_expanded_by_engine(self, client: AsyncClient) -> None:
+        headers = await login(client, "norm@example.com")
+        cid = await _create_conversation(client, headers)
+        resp = await client.post(
+            f"/v1/conversations/{cid}/messages",
+            json={
+                "content": [
+                    {"type": "markdown", "text": "说明\n```mermaid\nA-->B\n```\n$$\nz\n$$"}
+                ]
+            },
+            headers={**headers, "Idempotency-Key": "idem-norm"},
+        )
+        assert resp.status_code == 201, resp.text
+        blocks = resp.json()["message"]["content"]
+        assert [b["type"] for b in blocks] == ["markdown", "mermaid", "math"]
+        assert all(b["id"].startswith("block_") for b in blocks)
+        assert resp.json()["message"].get("parser_version") == "hunjing-markdown-v1"
+
+    async def test_python_cpp_golden_consistency(self) -> None:
+        """绑定输出与 C++ doctest 期望字节一致（跨端一致性最小 golden）。"""
+        from app.core import lingtai
+
+        out = lingtai.transform_markdown("x\n```mermaid\nA-->B\n```\n$$\nz\n$$\n")
+        assert out["parser_version"] == "hunjing-markdown-v1"
+        assert [b["type"] for b in out["blocks"]] == ["markdown", "mermaid", "math"]
+        # 与 engine/tests/test_transform.cpp 的确定性用例保持同一输入输出
+        out2 = lingtai.transform_markdown("x\n```mermaid\nA-->B\n```\n$$\nz\n$$\n")
+        assert out == out2
+
+    async def test_engine_failure_fails_closed(self, client: AsyncClient, monkeypatch) -> None:
+        from app.core import lingtai as lt
+
+        def boom(text: str):
+            raise lt.LingtaiUnavailable("灵台引擎调用失败：注入")
+
+        monkeypatch.setattr(lt, "transform_markdown", boom)
+        headers = await login(client, "fc@example.com")
+        cid = await _create_conversation(client, headers)
+        resp = await client.post(
+            f"/v1/conversations/{cid}/messages",
+            json={"content": [{"type": "markdown", "text": "hi"}]},
+            headers={**headers, "Idempotency-Key": "idem-fc"},
+        )
+        # fail closed：引擎不可用时显式失败，不静默降级直存
+        assert resp.status_code == 500