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/.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/