fix(shm): serialise concurrent writers with seq# + POSIX semaphore (fixes #195)

Two writers calling write_SM() on the same shmget() key used to
interleave strncpy() byte writes with no mutex, no semaphore, no
memory barrier, and no version counter - producing torn payloads
on the reader side.

Layout (any reader that does not know about the header will see a
leading integer and fail loudly, which is the desired signal):

    bytes [0..7]   uint64 little-endian sequence number
                   (odd = write in progress, even = ready, 0 = none)
    bytes [8..end] null-terminated payload

Protocol: a seqlock under a POSIX semaphore keyed by (shm_key + 1).
The writer acquires the lock, bumps seq# to odd, memcpy's the payload,
NUL-terminates, __ATOMIC_RELEASE fence, then bumps seq# back to even.
The reader does a seqlock-style read: load seq#1, copy payload,
__ATOMIC_ACQUIRE fence, load seq#2 - accept only if seq#1 == seq#2
and seq#1 is even.  Readers do not take the semaphore, so fan-out
stays cheap.

Mirrors the same change in concoredocker.hpp.  Adds:

  tests/test_shm_concurrency.py  Python regression (fork-based, Linux)
  tests/test_shm_cpp_harness.cpp fork-based C++ harness for CI
  tests/test_shm_cpp_smoke.cpp   header self-contained check

plus a cpp-shm-test job in .github/workflows/ci.yml that builds and
runs the harness on ubuntu-latest.

Author: concore-dev

.github/workflows/ci.yml

@@ -95,3 +95,20 @@ 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: Compile concore.hpp smoke test
+        run: g++ -std=c++17 -Wall -Wextra -c tests/test_shm_cpp_smoke.cpp -o /tmp/smoke.o
+
+      - name: Build C++ SHM harness
+        run: g++ -std=c++17 -O2 -Wall -Wextra -o /tmp/test_shm_cpp_harness tests/test_shm_cpp_harness.cpp
+
+      - name: Run C++ SHM harness
+        run: /tmp/test_shm_cpp_harness