feat(libmsg): extract pymsg-compatible messaging library as shared lib

- Add libmsg: C++20 header-only pymsg protocol implementation
  - Msg.hpp: pmsg_send/pmsg_recv (blocking + deadline modes) + MsgClient
  - MsgServer.hpp: multi-client TCP server with select event loop + thread pool
  - AsyncQueue<T>: generic async batch processing queue
  - RpcClient/RpcServer: JSON RPC convention layer (requires boost::json)
  - Msg_Basic.hpp/cpp: exception hierarchy, constants, version
- Fix thread safety: MsgClient _sockfd/_running use std::atomic with exchange()
- Fix deadline recv: client call() uses shared deadline for header+payload
- Fix graceful shutdown: thread pool drains queued tasks before exit
- Fix pmsg_send size check: enforce 64MiB limit (was 4GiB no-op check)
- Align AsyncQueue batch semantics with Python: batch_size is threshold
- Refactor gateway/LogC.hpp to use libmsg AsyncQueue<T>
- Update gateway Makefile to link -lus_msg, DbC.hpp to use <us/Msg.hpp>
- Add libmsg to _inst.sh build order

libmsg/inc/us/Msg_Basic.hpp

@@ -0,0 +1,79 @@
+#pragma once
+
+/** libmsg — UniServe 消息通信基础库
+ *
+ * 提供 pymsg 协议的 C++ 实现：4字节大端长度前缀 + UTF-8 载荷的 TCP 消息帧协议。
+ * 对应 Python 版 pymsg/pymsg/__init__.py，字节级兼容。
+ *
+ * 命名空间 pmc::net，库文件 libus_msg.so，头文件安装至 /usr/include/us/。
+ */
+
+#include <cstddef>
+#include <cstdint>
+#include <stdexcept>
+#include <string>
+
+#define MSG_CHECK(condition, message) \
+	if (!(condition)) { \
+		throw std::runtime_error(message); \
+	}
+
+namespace pmc {
+namespace net {
+
+/// 协议常量
+namespace msg_constants {
+    constexpr size_t   kHeaderSize  = 4;           ///< 帧头长度（字节）
+    constexpr uint32_t kMaxMsgSize  = 64u * 1024u * 1024u;  ///< 消息最大长度 64 MiB
+    constexpr int      kDefaultTimeout = 10;       ///< 默认超时（秒）
+    constexpr int      kSendTimeout    = 3;        ///< send() 默认超时（秒）
+    constexpr int      kListenPollMs   = 100'000;  ///< listen() select 轮询间隔（微秒）
+    constexpr double   kRetryLogEvery  = 5.0;      ///< 重试等待日志间隔（秒）
+}
+
+// ============================================================================
+// 异常层次
+// ============================================================================
+
+/// 消息库基础异常
+class MsgException : public std::runtime_error {
+public:
+    explicit MsgException(const std::string &msg) : std::runtime_error(msg) {}
+};
+
+/// 消息收发超时
+class MsgTimeoutException : public MsgException {
+public:
+    explicit MsgTimeoutException(const std::string &msg) : MsgException(msg) {}
+};
+
+/// 连接异常（断开、重置、地址错误等）
+class MsgConnectionException : public MsgException {
+public:
+    explicit MsgConnectionException(const std::string &msg) : MsgException(msg) {}
+};
+
+/// 目标服务未启动（ECONNREFUSED），用于 retry_on_refused 重试机制
+class MsgConnectionRefusedException : public MsgConnectionException {
+public:
+    explicit MsgConnectionRefusedException(const std::string &msg)
+        : MsgConnectionException(msg) {}
+};
+
+// ============================================================================
+// 基类（供 MsgClient 等内部工具方法继承使用）
+// ============================================================================
+
+class Msg_Basic {
+public:
+    class MsgExp : public std::runtime_error {
+    public:
+        MsgExp(const std::string& msg) : std::runtime_error(msg) {}
+    };
+protected:
+    /// 版本字符串（编译进 .so）
+    static const char* version();
+};
+
+} // namespace net
+} // namespace pmc