Fix first-open Tour fallback publication

[kvkenyon]

Summary

• Publish the already validated, persisted TourDocument before optional exact-artifact rendering.
• Keep rendering bounded and non-executable; surface material diagram categories with no validated diagram as unavailable.
• Cover first-open fallback visibility while rendering is held, fast tab re-entry, and the no-diagram state.

Validation

• swift run --package-path native TourWorkspaceFeatureTests
• scripts/native-check.sh
• npm run check