Fix YAML syntax error causing XML structure validation failure in sitemap build — a GitHub Copilot (GitHub) bot flight

From the pull request description

## Problem The GitHub Actions build was failing with the following error during sitemap validation: ``` 4️⃣ Testing XML structure... ❌ FAIL: Invalid XML structure Error: Process completed with exit code 1. ``` ## Root Cause The issue was caused by a YAML syntax error in the frontmatter of `calcu…

Excerpt (first 300 characters) — the full description is on GitHub.