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/Makefile

@@ -0,0 +1,39 @@
+VERSION := 0.0.1
+FRAMEWORK_NAME := us
+LIB_NAME := libus_msg
+INSTL_TDIR := /lib
+INCLU_TDIR := /usr/include
+INCLU_SDIR := inc
+CPP := g++
+INCS := -I inc
+CPPFLAGS := --std=c++20 -O3 -pipe -Wall -Werror
+LIBS :=
+BUILD_TDIR := build
+$(shell mkdir -p $(BUILD_TDIR))
+
+#--------
+all: $(BUILD_TDIR)/$(LIB_NAME).so
+
+$(BUILD_TDIR)/$(LIB_NAME).so: $(BUILD_TDIR)/Msg_Basic.o
+	$(CPP) -shared $^ $(LIBS) -o $@
+
+$(BUILD_TDIR)/Msg_Basic.o: src/Msg_Basic.cpp
+	$(CPP) $(CPPFLAGS) -fPIC -c $^ $(INCS) -o $@
+
+clear:
+	@rm -rvf $(BUILD_TDIR)
+
+install:
+	@echo "install UniServe_libmsg..."
+	@mkdir -vp $(INCLU_TDIR)/$(FRAMEWORK_NAME)
+	@cp -vf $(INCLU_SDIR)/$(FRAMEWORK_NAME)/Msg_Basic.hpp $(INCLU_TDIR)/$(FRAMEWORK_NAME)/
+	@cp -vf $(INCLU_SDIR)/$(FRAMEWORK_NAME)/Msg.hpp $(INCLU_TDIR)/$(FRAMEWORK_NAME)/
+	@cp -vf $(BUILD_TDIR)/$(LIB_NAME).so $(INSTL_TDIR)/
+	@echo "done."
+
+uninstall:
+	@echo "uninstall UniServe_libmsg..."
+	@rm -vf $(INCLU_TDIR)/$(FRAMEWORK_NAME)/Msg_Basic.hpp
+	@rm -vf $(INCLU_TDIR)/$(FRAMEWORK_NAME)/Msg.hpp
+	@rm -vf $(INSTL_TDIR)/$(LIB_NAME).so
+	@echo "done."