ENH: Add platform-specific package install inputs and OS selection

Add optional inputs to the CXX build workflow:

- apt-packages: Install system packages on Linux (apt-get)
- brew-packages: Install system packages on macOS (brew)
- choco-packages: Install system packages on Windows (choco)
- os-list: JSON array of runner OS labels to build on, allowing
  modules to disable platforms that cannot work (e.g., skip Windows
  when the dependency has no Windows package)

Default os-list includes all platforms for backwards compatibility:
  ["ubuntu-22.04", "windows-2022", "macos-15-intel", "macos-15"]

Usage to skip Windows:
```yaml
cxx-build-workflow:
  uses: .../build-test-cxx.yml@v5.4.6
  with:
    apt-packages: 'libopenslide-dev'
    brew-packages: 'openslide'
    os-list: '["ubuntu-22.04", "macos-15-intel", "macos-15"]'
```

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hjmjohnson

.github/workflows/build-test-cxx.yml

@@ -30,14 +30,31 @@ on:
         # example: MeshToPolyData@3ad8f08:BSplineGradient@0.3.0
         required: false
         type: string
+      apt-packages:
+        description: 'Space-separated list of apt packages to install on Linux runners before building (e.g. libopenslide-dev libfftw3-dev)'
+        required: false
+        type: string
+      brew-packages:
+        description: 'Space-separated list of Homebrew packages to install on macOS runners before building (e.g. openslide libomp)'
+        required: false
+        type: string
+      choco-packages:
+        description: 'Space-separated list of Chocolatey packages to install on Windows runners before building'
+        required: false
+        type: string
+      os-list:
+        description: 'JSON-formatted array of runner OS labels to build on (default: all platforms)'
+        required: false
+        type: string
+        default: '["ubuntu-22.04", "windows-2022", "macos-15-intel", "macos-15"]'
 
 jobs:
   build-test-cxx:
     runs-on: ${{ matrix.os }}
     strategy:
       max-parallel: 3
       matrix:
-        os: [ubuntu-22.04, windows-2022, macos-15-intel, macos-15]
+        os: ${{ fromJSON(inputs.os-list) }}
         include:
           - os: ubuntu-22.04
             c-compiler: "gcc"
@@ -65,6 +82,22 @@ jobs:
       with:
         large-packages: false
 
+    - name: Install system packages (Linux)
+      if: inputs.apt-packages != '' && runner.os == 'Linux'
+      run: |
+        sudo apt-get update -qq
+        sudo apt-get install -y -qq ${{ inputs.apt-packages }}
+
+    - name: Install system packages (macOS)
+      if: inputs.brew-packages != '' && runner.os == 'macOS'
+      run: |
+        brew install ${{ inputs.brew-packages }}
+
+    - name: Install system packages (Windows)
+      if: inputs.choco-packages != '' && runner.os == 'Windows'
+      run: |
+        choco install -y ${{ inputs.choco-packages }}
+
     - name: Set up Python 3.11
       uses: actions/setup-python@v5
       with: