- Create notebooks/00_START_HERE.ipynb as the single entry point with plan
descriptions, audience guidance, and links to all 4 plans
- Add navigation footer cells to all 11 content notebooks with Next/Previous
links and back-link to Start Here
- Terminal notebooks (plan endings) offer cross-plan links to explore other plans
- Plan C dashboard gets explicit recommended reading order (Track A → B → C)
- Add test_start_here_exists_and_links_all_plans and
test_every_notebook_has_navigation_footer to test suite
- Skip navigation-only notebooks in code-cell and assessment tests
