[Good first issue] Add one source-neutral DESIGN.md theme candidate example

[ch040602]

Goal:
Add one small source-neutral DESIGN.md example and its approval-bound mdpr-theme-candidate-v1 output.

Scope:

• The DESIGN.md should describe best-fit scenarios, semantic layout blueprints, and decoration grammar.
• The generated candidate must remain proposal-only and identify MDPR registration targets.
• Do not claim the theme is available in MDPR runtime until import/approval gates exist.

Evidence:

• DESIGN.md excerpt or fixture path.
• Candidate JSON path if generated.
• Schema or CLI validation command.

Boundary:
No copied layouts, private source content, exact slide copy, final coordinates, exact icon choices, renderer object IDs, or runtime theme-binding claims.