[Good first issue] Add one public Markdown corpus case

[ch040602]

Goal:
Add one public Markdown source or compact fixture that stresses mdpr-skill visual evaluation coverage.

Scope:

• Pick a public Markdown source with clear license/provenance.
• Explain the stress point: nested lists, dense tables, code-heavy sections, diagrams, citations, or section splitting.
• Add the smallest fixture or documented corpus entry needed for review.

Evidence:

• Link the source.
• Name the expected review criterion or visual risk.
• Run the focused test or explain why it is documentation-only.

Boundary:
Do not ask mdpr-skill to choose final coordinates, exact colors, z-order, renderer object IDs, or exact icon assets. Route parser/layout/PPTX runtime bugs to MDPR.