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/.gitignore

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+# Prerequisites
+*.d
+
+# Compiled Object files
+*.slo
+*.lo
+*.o
+*.obj
+
+# Precompiled Headers
+*.gch
+*.pch
+
+# Linker files
+*.ilk
+
+# Debugger Files
+*.pdb
+
+# Compiled Dynamic libraries
+*.so
+*.dylib
+*.dll
+*.so.*
+
+
+# Fortran module files
+*.mod
+*.smod
+
+# Compiled Static libraries
+*.lai
+*.la
+*.a
+*.lib
+
+# Executables
+*.exe
+*.out
+*.app
+
+# Build directories
+build/
+Build/
+build-*/
+
+# CMake generated files
+CMakeFiles/
+CMakeCache.txt
+cmake_install.cmake
+#Makefile
+install_manifest.txt
+compile_commands.json
+
+# Temporary files
+*.tmp
+*.log
+*.bak
+*.swp
+
+# vcpkg
+vcpkg_installed/
+
+# debug information files
+*.dwo
+
+# test output & cache
+Testing/
+.cache/