feat(libmsg): add pymsg C++ library with AsyncQueue, MsgServer, and JSON RPC layer

- Add libmsg module: pymsg protocol implementation (4-byte BE length prefix + UTF-8)
- Add AsyncQueue<T>: generic async batch queue with backpressure (aligned with Python AsyncRpcQueue)
- Add MsgServer: select-based multi-client TCP server with optional thread pool
- Add RpcClient/RpcServer: JSON RPC convention layer with automatic route injection
- Refactor gateway/LogC.hpp to use AsyncQueue for async batch log delivery
- Update gateway/DbC.hpp to add call_msg() using pymsg TCP protocol
- Remove duplicated gateway/inc/Msg.hpp (now provided by libmsg)
- Update gateway/Makefile and _inst.sh build order

Fixes included:
- pmsg_send size check uses kMaxMsgSize (64MiB) instead of 4GiB
- MsgClient._running uses std::atomic<bool> for thread safety
- Deadline-based recv timeout to prevent Slowloris (header+payload shared deadline)

libmsg/LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 建昌风物大顺脚本
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.