fix(shm): serialise concurrent writers with seq# + POSIX semaphore

.github/workflows/ci.yml

@@ -96,3 +96,18 @@ jobs:
       - name: Validate Dockerfile build
         if: steps.filter.outputs.dockerfile == 'true'
         run: docker build -f Dockerfile.py -t concore-py-test .
+
+  cpp-shm-test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install g++
+        run: sudo apt-get update && sudo apt-get install -y g++
+
+      - name: Parse concore.hpp standalone
+        # -fsyntax-only parses concore.hpp without producing an object
+        # file, so we don't need system SHM semaphores or shared-memory
+        # segments to succeed. This catches header-level regressions on
+        # the SHM transport (issue #195) without needing g++ at runtime.
+        run: g++ -std=c++17 -Wall -Wextra -fsyntax-only tests/test_shm_cpp_smoke.cpp