C++ ZMQ transport doesn't compile (zmq.hpp included inside namespace concore_base) #632

Building a C++ node with -DCONCORE_USE_ZMQ fails on dev, for both concore.hpp and concoredocker.hpp. concore_base.hpp includes <zmq.hpp> inside namespace concore_base, so the standard headers it pulls in end up in that namespace and g++ stops with ~740 errors:

In file included from /usr/include/zmq.hpp:118,
                 from concore_base.hpp:411,
                 from concore.hpp:35,
                 from my_node.cpp:1:
/usr/include/c++/14/array:62:29: error: '__is_swappable' does not name a type

Nothing in CI builds the C++ side with ZMQ turned on, so it went unnoticed.

Once it builds, the C++ REQ socket also has the problem Java (#577) and Python (#599) had: after one missed reply every later send fails with "Operation cannot be accomplished in current state".

I have a fix with tests and a CI step for this, I'll open a PR.