✨ 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/app/conversations/service.py

@@ -14,6 +14,7 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.conversations.schemas import MessageIn
+from app.core import lingtai
 from app.core.ids import new_id
 from app.models import (
     AuditEvent,
@@ -122,15 +123,25 @@ async def replay_or_execute_message(
     conversation = await _require_conversation(db, domain_id, conversation_id)
     branch = await _get_default_branch(db, conversation.id)
 
-    blocks: list[dict[str, Any]] = [
-        {**b.model_dump(exclude_none=True), "id": new_id("block_"), "status": "completed"}
-        for b in body.content
-    ]
+    blocks: list[dict[str, Any]] = []
+    parser_version: str | None = None
+    for b in body.content:
+        if b.type == "markdown":
+            # 契约第 4 节：markdown 输入经确定性 parser 规范化为 canonical blocks
+            normalized = lingtai.transform_markdown(b.text)
+            parser_version = str(normalized["parser_version"])
+            blocks.extend(normalized["blocks"])
+        else:
+            blocks.append({**b.model_dump(exclude_none=True), "status": "completed"})
+    for block in blocks:
+        block["id"] = new_id("block_")
+        block.setdefault("status", "completed")
     message = Message(
         conversation_id=conversation.id,
         branch_id=branch.id,
         role="user",
         content=blocks,
+        parser_version=parser_version,
     )
     db.add(message)
     await db.flush()