build: portability groundwork toward a Windows/MSVC viewer

- CMakeLists.txt: fetch libzmq at the top level (zeromq/libzmq v4.3.5) before
  slsDetectorPackage, so this project controls the ZeroMQ version instead of sls's
  bundled archive. sls reuses it via its if(NOT libzmq_POPULATED) guard, so a single
  libzmq-static target is built (no duplicate-target/double-symbol clash). Verified the
  full Linux build still links (JFJochZMQ -> JFJochReceiver -> jfjoch_process).
- common/NetworkAddressConvert.cpp: guard the network includes for _WIN32
  (winsock2/ws2tcpip vs arpa/inet).
- common/ImageBuffer.cpp: use std::malloc/std::free for the non-NUMA path instead of an
  anonymous mmap (the mapping had no huge-page/mbind flags, so it was equivalent) -
  portable and removes the POSIX-only dependency. The NUMA-interleave path is unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

common/NetworkAddressConvert.cpp

@@ -1,7 +1,12 @@
 // SPDX-FileCopyrightText: 2024 Filip Leonarski, Paul Scherrer Institute <filip.leonarski@psi.ch>
 // SPDX-License-Identifier: GPL-3.0-only
 
+#ifdef _WIN32
+#include <winsock2.h>
+#include <ws2tcpip.h>
+#else
 #include <arpa/inet.h>
+#endif
 #include <sstream>
 #include <iomanip>