MAINT: Phase 1 - Add HTML archives to GitHub release assets (#422)

Implements Phase 1: HTML archive backup on releases.

## Changes

This commit adds steps to the publish.yml workflow to create and upload HTML archives as GitHub release assets:

1. Create HTML archive - Compresses _build/html/ into a .tar.gz archive after the HTML build
2. Generate checksum - Creates SHA256 hash for integrity verification (html-checksum.txt)
3. Create manifest - Generates metadata file with build information (html-manifest.json)
4. Upload to release - Attaches all three files to the GitHub release using softprops/action-gh-release@v1

## Release Assets Created

Each publish-* tag will now include:

• 📦 lecture-python-programming-html-{tag}.tar.gz - Full HTML site archive
• 🔐 html-checksum.txt - SHA256 verification file
• 📋 html-manifest.json - Build metadata (tag, commit, timestamp, size, file count)

## Key Features

• ✅ Does not modify _build/html/ directory (gh-pages deployment unaffected)
• ✅ Preserves existing release notes (no body override)
• ✅ Automatic tag detection from workflow context
• ✅ Creates safety net before Phase 2 (gh-pages history cleanup)

Author: mmcky

.github/workflows/publish.yml

@@ -75,6 +75,32 @@ jobs:
         shell: bash -l {0}
         run: |
           jb build lectures --path-output ./ -n -W --keep-going
+      # Create HTML archive for release assets
+      - name: Create HTML archive
+        shell: bash -l {0}
+        run: |
+          tar -czf lecture-python-programming-html-${{ github.ref_name }}.tar.gz -C _build/html .
+          sha256sum lecture-python-programming-html-${{ github.ref_name }}.tar.gz > html-checksum.txt
+
+          # Create metadata manifest
+          cat > html-manifest.json << EOF
+          {
+            "tag": "${{ github.ref_name }}",
+            "commit": "${{ github.sha }}",
+            "timestamp": "$(date -Iseconds)",
+            "size_mb": $(du -sm _build/html | cut -f1),
+            "file_count": $(find _build/html -type f | wc -l)
+          }
+          EOF
+      - name: Upload archives to release
+        uses: softprops/action-gh-release@v1
+        with:
+          files: |
+            lecture-python-programming-html-${{ github.ref_name }}.tar.gz
+            html-checksum.txt
+            html-manifest.json
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       - name: Deploy website to gh-pages
         uses: peaceiris/actions-gh-pages@v4
         with: