Astro drafts skip build: flip draft to validate MDX diagrams

In an Astro content collection where draft: true posts are filtered out of the build, bun run build passing says nothing about the draft itself: its MDX and build-time Mermaid diagrams are never compiled. To validate, temporarily set draft: false, build, check dist/blog/<slug>/index.html (e.g. count <svg for rendered Mermaid), then set it back and rebuild so dist/ does not keep a copy of the unpublished post. Separately: a smoke test asserting a fixed scenario count (n == 5) broke CI when a new scenario was added; compare against the reference run's length instead.