docs(DESIGN.md): align with actual CSS

- Expand --bg-page surface entry to document the vertical gradient
  (--bg-page-top, --bg-page, --bg-page-end) and top-left radial highlight
- Add --hero-bg-mid to the hero gradient token description
- Clarify non-interactive element token usage: ink tokens on --bg-paper-strong
  or --bg-paper; call out specific .source-badge and .sponsorship-body code rules
- Replace vague 1px shadow rule with the two allowed depth treatments:
  search inset/shadow and primary CTA shadow
- Correct 0.4rem radius callout from 'expand-row' to '.sponsorship-body code'
- Update hero gradient in Depth section to include --hero-bg-mid
- Update .tag hover description: --highlight bg, --tag-hover-border border,
  ink text; active state adds hero-ink text; rename variants to tag-group/tag-source
- Update Source badge / inline code component entry to reflect --ink-soft vs --ink
- Tighten Iteration Guide audit items 2 and 3 to match the above

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