ci: add matrix workflow including windows to catch platform-specific test flakes

.github/workflows/ci-matrix.yml

@@ -0,0 +1,45 @@
+name: CI Matrix (including Windows)
+
+on:
+  pull_request:
+    branches: [ main ]
+  push:
+    branches: [ main ]
+
+jobs:
+  test:
+    name: Test on ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+        python-version: [3.11]
+
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt || true
+      - name: Run tests
+        run: |
+          # ensure a project-local temp dir to avoid AppData permission issues on Windows
+          python - <<'PY'
+import os, pathlib
+p = pathlib.Path('tmp')
+p.mkdir(exist_ok=True)
+os.environ['TMP'] = str(p.resolve())
+os.environ['TEMP'] = str(p.resolve())
+print('TMP/TEMP set to', os.environ['TMP'])
+PY
+          pytest --basetemp=tmp/pytest --cov --cov-branch --cov-report=xml
+      - name: Upload coverage to Artifacts
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage-xml-${{ matrix.os }}
+          path: coverage.xml