Fix sticky dark header contrast

[paladini]

Summary

• keep the shared product header on its light paper background after scrolling in dark mode
• reuse the existing --hs-header-paper token through VitePress's --vp-nav-bg-color
• add a patch changeset for the user-facing docs fix

Root cause

VitePress removes the .top class from the home navigation after scrolling. Its more-specific sticky navigation selector then reapplies the dark --vp-nav-bg-color, while the Harness Score wordmark, navigation links, and controls retain their dark ink colors.

By setting VitePress's navigation background token to the existing Harness Score paper color, the built-in top, sticky, sidebar, and mobile states all resolve to the intended header background without new specificity overrides or !important declarations.

Browser verification

Verified locally with Playwright:

• dark mode at 1904x993, both at the top and at scrollY=600
• internal guide page with sidebar
• light mode
• mobile 390x844 with navigation closed and open

In the reported desktop dark-mode scenario, the sticky header now remains rgb(241, 244, 236) with the wordmark and main links at rgb(13, 27, 23).

Validation

• npm run docs:build
• npm test — 268 tests passed
• npm run lint — passed with 18 pre-existing warnings and no errors
• npm run scan -- --min-level 4 — L4, 108/108
• git diff --check