Canvas and paper were both #ffffff in light mode — cards only separated
by a 1px hairline, causing eye fatigue. Tints the canvas token to a
very pale cool neutral (#f3f5f8) so paper cards lift naturally without
shadow or radius, preserving brutalism. Dark mode tokens unchanged.
Updates DESIGN.md Surface section with rationale.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
