feat(mint): add contrastRatio and failsWCAG fields to ColorCluster type

[nujovich]

Card: https://github.com/nujovich/mint-radar/issues/20

What: Adds WCAG contrast-ratio awareness to Mint's CSS audit. Milestone 1 extended the ColorCluster interface with contrastRatio and failsWCAG fields. Milestone 2 implements the WCAG 2.1 contrast ratio calculator in lib/css-contrast.mjs (parseSrgbColor, relativeLuminance, contrastRatio, evaluateContrast, annotateColorClusters) and wires it into the CLI audit path (bin/mint-ds.mjs) and the playground API (app/api/audit/route.ts) so every color cluster is automatically annotated with its contrast ratio and AA/AAA pass flags against the detected background. The calculator supports hex, rgb(), hsl(), oklch(), and oklab() literals with wide-gamut fallback to sRGB. Milestone 3 adds buildContrastPairs() that generates a dedicated contrastPairs report on AuditReport listing foreground/background pairs that fail WCAG AA or AAA, each with a contrast-color() migration suggestion.

Why: 83.9% of homepages fail contrast (WebAIM Million 2026). Mint's CSS audit detects color clusters but had no awareness of accessibility contrast requirements. Milestone 2 computes WCAG 2.1 ratios for detected color-background pairs automatically after every audit, annotating each ColorCluster with its contrast ratio and WCAG pass/fail flags. Milestone 3 surfaces failing pairs in a dedicated contrastPairs report with actionable contrast-color() migration suggestions.

Milestones

• Milestone 1 -- Extend ColorCluster in lib/types.ts with contrastRatio (number) and failsWCAG ({ aa: boolean; aaa: boolean }) fields
• Milestone 2 -- Implement WCAG 2.1 contrast ratio calculation for each color-background pair detected in the audited CSS
• Milestone 3 -- Add failing contrast pairs report in AuditReport with contrast-color() migration suggestions
• Milestone 4 -- Add tests with WebAIM Million baseline (83.9% fail rate)

How to test:

npx vitest run lib/__tests__/css-contrast.test.mjs
# Full suite:
npm test

[nujovich]

All milestones complete. Ready for review when you are.