- packages/registry/src/code-group.tsx: new component; reads data-title/
  data-language from child props (set by rehype-pretty-code), renders a
  tabbed figure with keyboard navigation (ArrowLeft/Right/Home/End) and
  a per-tab copy button
- registry/mdx/code-group.json + registry/registry.json: regenerated via
  build:registry (23 components total)
- scripts/build-registry.ts: added code-group metadata
- CLI add.ts: code-group entry in COMPONENT_MDX_MAP
- CLI update.ts: code-group.tsx in FILE_TO_COMPONENT
- www: code-group.tsx synced, mdx-components.tsx wired, sidebar entry and
  doc page added
- Bump version 0.0.19 → 0.0.20; update component counts to 23 in READMEs