ci: configure multi-OS testing matrix for Linux, Windows, and macOS

.github/workflows/ci.yml

@@ -11,8 +11,13 @@ permissions:
 
 jobs:
   validate:
-    name: Lint & Verify
+    name: Lint & Verify (${{ matrix.os }}, Python ${{ matrix.python-version }})
-    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+        python-version: ["3.11", "3.12"]
+    runs-on: ${{ matrix.os }}
     timeout-minutes: 10
 
     steps:
@@ -24,7 +29,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
-          python-version: "3.11"
+          python-version: ${{ matrix.python-version }}
 
       - name: Verify accessible SVG contract
         run: python3 scripts/test-lint-a11y.py