Add local DESIGN.md preview compiler

Problem

The repository documents design tokens but does not offer a local way to inspect their visual language. Current previews live outside this repository.

Proposal

Add a dependency-free local preview compiler that renders a brand-faithful design-system showcase from each DESIGN.md.

Acceptance criteria

• npm run preview builds every preview and serves an index.
• npm run preview -- <name> builds and serves one named design directly.
• Rendered previews use the source document's palette, typography, component recipes, spacing, and radius tokens.
• Structured YAML and legacy prose-only documents both work.
• Generated HTML stays out of version control and can be recreated on demand.

Implementation

A fork branch implements this with Node built-ins only, focused test coverage, and README instructions.