[DOCUMENTATION] Updates CI docs with some notes on CI variables. (- WIP PR #328 -)

Changes in file .github/workflows/CI-BUILD.yml:
  * Removes env.PYTHON_DEFAULT shadowing
  * Related work (- WIP #254 -)

Changes in file docs/CI.md:
  * Documents new variables for #254 (- WIP PR #328 -)
  * Adds a few new badges (- WIP #320 -)
  * Implements #326

Author: reactive-firewall

.github/workflows/CI-BUILD.yml

@@ -51,7 +51,7 @@ jobs:
           persist-credentials: false
       - uses: actions/setup-python@0b93645e9fea7318ecaed2b359559ac225c90a2b # v5.3.0
         with:
-          python-version: ${{ env.PYTHON_DEFAULT }}
+          python-version: "${{ vars.PYTHON_DEFAULT }}"
       - name: Pre-Clean
         id: clean
         run: make -j1 -f Makefile purge 2>/dev/null || true
@@ -86,24 +86,24 @@ jobs:
         lang-var: ["de.utf-8", "jp.utf-8"]
         experimental: [true]
         include:
-          - python-version: "3.10"
+          - python-version: "${{ vars.PYTHON_DEFAULT }}"
             lang-var: "de.utf-8"
             experimental: false
-          - python-version: "3.10"
+          - python-version: "${{ vars.PYTHON_DEFAULT }}"
             lang-var: "jp.utf-8"
             experimental: false
-          - python-version: "3.10"
+          - python-version: "${{ vars.PYTHON_OLD_MIN }}"
             lang-var: "en_US.utf-8"
             experimental: false
-          - python-version: "3.11"
+          - python-version: "${{ vars.PYTHON_OLD_EXTRA }}"
             lang-var: "en_US.utf-8"
             experimental: false
           - python-version: "${{ vars.PYTHON_DEFAULT }}"
             lang-var: "en_US.utf-8"
             experimental: false
-          - python-version: "3.12"
+          - python-version: "${{ vars.PYTHON_EXPERIMENTAL }}"
             lang-var: "en_US.utf-8"
-            experimental: false
+            experimental: ${{ (needs.BUILD.environment == 'Experimenting') }}
     outputs:
       bootstrap_status: ${{ steps.bootstrap.outcome }}
     env:

docs/CI.md

@@ -11,6 +11,55 @@ Continuous integration testing is handled by GitHub Actions and the generous Cir
 [![CodeCov](https://codecov.io/gh/reactive-firewall/multicast/graphs/sunburst.svg)](https://codecov.io/gh/reactive-firewall/multicast)
 [![OpenSSF Best Practices](https://www.bestpractices.dev/projects/9458/badge)](https://app.deepsource.com/report/3f7c11ca-c203-44ec-9e01-126078f11660)
 [GHA Metrics](https://github.com/reactive-firewall/multicast/actions/metrics/performance?dateRangeType=DATE_RANGE_TYPE_LAST_90_DAYS)
+![CodeRabbit Pull Request Reviews](https://img.shields.io/coderabbit/prs/github/reactive-firewall/multicast?utm_source=oss&utm_medium=github&utm_campaign=reactive-firewall%2Fmulticast&labelColor=171717&color=FF570A&link=https%3A%2F%2Fcoderabbit.ai&label=CodeRabbit+Reviews)
+[![Average time to resolve an issue](https://isitmaintained.com/badge/resolution/reactive-firewall/multicast.svg)](https://isitmaintained.com/project/reactive-firewall/multicast "Average time to resolve an issue")
+[![Percentage of issues still open](https://isitmaintained.com/badge/open/reactive-firewall/multicast.svg)](https://isitmaintained.com/project/reactive-firewall/multicast "Percentage of issues still open")
+
+## Configurable CI Variables
+
+This section documents environment variables used across CI workflows to ensure consistency and
+simplify maintenance.
+
+### Python Version Variables
+
+To standardize Python version management across all CI workflows, we use a set of environment
+variables defined at the top of each workflow file. This approach centralizes version definitions
+while maintaining separation between workflows, making future updates easier and ensuring
+consistency.
+
+#### Standard Python Version Variables
+
+| Variable | Purpose | Example Value |
+|----------|---------|---------------|
+| `PYTHON_DEFAULT` | The default Python version used for single-version jobs | `"3.12"` |
+| `PYTHON_OLD_MIN` | Oldest/minimum Python version for cross-python portability testing | `"3.10"` |
+| `PYTHON_OLD_EXTRA` | Additional Python version for coverage testing | `"3.11"` |
+| `PYTHON_EXPERIMENTAL` | Future/experimental Python version for optional testing | `"3.13"` |
+
+#### Usage Examples
+
+##### Setting up Python with the default version
+
+```yaml
+- uses: actions/setup-python@v5
+  with:
+    python-version: "${{ vars.PYTHON_DEFAULT }}"
+```
+
+##### Using matrix strategy for multi-version testing
+
+```yaml
+jobs:
+  test:
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version: ["${{ vars.PYTHON_DEFAULT }}", "${{ vars.PYTHON_EXPERIMENTAL }}"]
+    steps:
+      - uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+```
 
 ## Testing