✨ feat: WP-03 三界归真作用域与消息版本谱系（ADR-020 落地）

- 迁移 5：conversation.realm（默认真界，检查约束）；message.realm 快照、canonized_at、parent_message_id 自引用与 variant_kind（origin/revision/candidate）
- 界域端点：GET conversation（realm/当前分支）与 POST realm（入境/出境显式切换，留审计——ADR-020 切换必须可见）
- 归真端点 POST /v1/messages/{id}/canonizations：scope 必须与消息界域一致（不一致 422）；幂等（重复归真 200 原状态）；境界归真产出 world.canonized Outbox 事件（世界知识候选入口，WP-04 消费）；幻境归真仅标记试演记录
- 编辑版本 POST /v1/messages/{id}/revisions：新消息 variant=revision 引用原消息（markdown 修订同样经灵台引擎规范化），原消息永不修改
- 测试 44 项全绿（+8）：界域快照、归真正/反/幂等、幻境试演、revision 原消息不变、跨域 401/403/404 fail-closed

backend/app/conversations/routes.py

@@ -206,3 +206,96 @@ async def set_current_branch(
         db, domain.id, conversation_id, body.get("branch_id", "")
     )
     return {"id": conversation.id, "current_branch_id": conversation.current_branch_id or ""}
+
+
+@router.get("/{conversation_id}")
+async def get_conversation(
+    conversation_id: str,
+    db: DbSession,
+    user: Annotated[User, Depends(get_current_user)],
+) -> dict[str, object]:
+    domain = await _domain_of(db, user)
+    conversation = await service.require_conversation(db, domain.id, conversation_id)
+    from app.conversations.schemas import ConversationOut
+
+    return ConversationOut(
+        id=conversation.id,
+        title=conversation.title,
+        current_branch_id=conversation.current_branch_id,
+        created_at=conversation.created_at.isoformat(),
+    ).model_dump() | {"realm": conversation.realm}
+
+
+@router.post("/{conversation_id}/realm")
+async def set_realm(
+    conversation_id: str,
+    db: DbSession,
+    user: Annotated[User, Depends(require_state_change)],
+    request: Request,
+    body: dict[str, str],
+) -> dict[str, str]:
+    """入境/出境：切换对话界域（显式、留审计）。"""
+    domain = await _domain_of(db, user)
+    conversation = await service.set_realm(
+        db,
+        domain.id,
+        conversation_id,
+        body.get("realm", ""),
+        getattr(request.state, "trace_id", None),
+    )
+    return {"id": conversation.id, "realm": conversation.realm}
+
+
+message_router = APIRouter(prefix="/v1/messages", tags=["messages"])
+
+
+@message_router.post("/{message_id}/canonizations", status_code=201, response_model=None)
+async def canonize_message(
+    message_id: str,
+    db: DbSession,
+    user: Annotated[User, Depends(require_state_change)],
+    request: Request,
+    body: dict[str, str],
+) -> JSONResponse | dict[str, str | None]:
+    """归真：携带 canonical_scope（ADR-020）；重复归真幂等返回原状态（200）。"""
+    from fastapi.responses import JSONResponse
+
+    domain = await _domain_of(db, user)
+    message, replayed = await service.canonize_message(
+        db, domain.id, message_id, body.get("scope", ""), getattr(request.state, "trace_id", None)
+    )
+    payload = {
+        "message_id": message.id,
+        "canonical_scope": message.realm,
+        "canonized_at": message.canonized_at.isoformat() if message.canonized_at else None,
+    }
+    if replayed:
+        return JSONResponse(status_code=200, content=payload)
+    return payload
+
+
+@message_router.post("/{message_id}/revisions", status_code=201)
+async def revise_message(
+    message_id: str,
+    db: DbSession,
+    user: Annotated[User, Depends(require_state_change)],
+    request: Request,
+    body: MessageIn,
+) -> dict[str, object]:
+    """编辑版本：新消息引用原消息；原消息永不修改。"""
+    domain = await _domain_of(db, user)
+    revision = await service.revise_message(
+        db, domain.id, message_id, body, getattr(request.state, "trace_id", None)
+    )
+    from app.conversations.schemas import MessageOut
+
+    return MessageOut(
+        id=revision.id,
+        conversation_id=revision.conversation_id,
+        branch_id=revision.branch_id,
+        role=revision.role,
+        content=revision.content,
+        parser_version=revision.parser_version,
+        created_at=revision.created_at.isoformat(),
+    ).model_dump() | {"parent_message_id": revision.parent_message_id,
+                      "variant_kind": revision.variant_kind}