Publish layered DCB student notebook and colour-coded setup

[allamaprabhuani]

Summary

• Add the student notebook with editable geometry, material properties, boundary conditions, solver settings, and retained-result interpretation. Full fracture simulation remains opt-in.
• Add a source-driven material/BC diagram and a plot-only command using the existing PhAST mesh, assignment, and selection routines.
• Distinguish three bulk regions, equal elastic moduli, differing fracture toughness, and prescribed inputs from computed damage. Preserve existing numerical results and validation limitations.
• Link the notebook and diagram from the documentation and example READMEs. New heterogeneous calculation outputs also include the input diagram.

Checks completed

• 149 focused workflow, example-contract, and teaching-notebook tests passed.
• Strict Sphinx HTML build passed with PHAST_NB_EXECUTION_MODE=force; all six teaching notebooks executed, including the layered DCB notebook in 4.34 seconds using retained results.
• All 11 local images on the built student page resolved, including the setup PNG and retained crack GIF.
• Setup figure visually inspected; generation uses no fracture solve.
• Independent read-only review found no source-level release blocker. It noted a relative custom-output-path caveat and Bash command-continuation ergonomics; the tested default is unaffected.

No new full DCB simulation or physical validation is claimed. Private notes and raw trajectory stores are excluded. The public-push safety acknowledgement covers only intentional, generic OneDrive synchronisation advice, with no personal paths or private data.