docs(DESIGN.md): add YAML frontmatter, hex approximations, typography table, and Iteration Guide

- Add YAML frontmatter with version, name, and description
- Annotate each color token with approximate hex sRGB equivalent
- Replace bullet-list type scale with a structured typography table
- Add Iteration Guide section with six-step post-generation audit checklist
- Add Known Gaps section documenting color format, frontmatter scope, and spacing scale

Co-Authored-By: Claude <noreply@anthropic.com>