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/Makefile

@@ -0,0 +1,47 @@
+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 $(INCLU_SDIR)/$(FRAMEWORK_NAME)/MsgServer.hpp $(INCLU_TDIR)/$(FRAMEWORK_NAME)/
+	@cp -vf $(INCLU_SDIR)/$(FRAMEWORK_NAME)/AsyncQueue.hpp $(INCLU_TDIR)/$(FRAMEWORK_NAME)/
+	@cp -vf $(INCLU_SDIR)/$(FRAMEWORK_NAME)/RpcClient.hpp $(INCLU_TDIR)/$(FRAMEWORK_NAME)/
+	@cp -vf $(INCLU_SDIR)/$(FRAMEWORK_NAME)/RpcServer.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 $(INCLU_TDIR)/$(FRAMEWORK_NAME)/MsgServer.hpp
+	@rm -vf $(INCLU_TDIR)/$(FRAMEWORK_NAME)/AsyncQueue.hpp
+	@rm -vf $(INCLU_TDIR)/$(FRAMEWORK_NAME)/RpcClient.hpp
+	@rm -vf $(INCLU_TDIR)/$(FRAMEWORK_NAME)/RpcServer.hpp
+	@rm -vf $(INSTL_TDIR)/$(LIB_NAME).so
+	@echo "done."