feat(mint): add modern CSS feature adoption scan to audit

[nujovich]

Card: https://github.com/nujovich/mint-radar/issues/13

What: Adds a modern CSS feature adoption scan (STEP 14) to Mint's audit pipeline. The scan detects presence or absence of eight modern CSS features (@Property, @layer, @container, @supports, native nesting, color-mix(), @scope, :has()) and provides adoption suggestions for features not yet used. Results are reported in a new modernFeatures field on the AuditReport.

Why: Project Wallace 2026 revealed critically low adoption of modern CSS features (@Property 2.67%, @layer 2.71%, @container 9.61%). Mint already scores CSS health (chaos score) but lacks awareness of which modern features a codebase uses and which it should adopt. This scan complements the chaos score by giving developers an actionable roadmap toward modern CSS.

Milestones

• Milestone 1 -- Extend audit prompt in lib/prompts.mjs with STEP 14: modern CSS feature scan (detect @Property, @layer, @container, @supports, nesting, color-mix, @scope, :has()), report presence/absence with adoption suggestions
• Milestone 2 -- Add modernFeatures field to AuditReport type in lib/types.ts
• Milestone 3 -- Wire modern features results in playground UI as "Modern CSS Adoption" section
• Milestone 4 -- Add test fixture with modern and legacy CSS and tests in lib/tests/

Milestone 4 detail

• lib/fixtures/modern-features.css -- fixture exercising all eight tracked features plus a legacy block with none of them.
• lib/tests/prompts.test.mjs -- buildAuditPrompt with the modern features fixture block: embeds the fixture and preserves modern anchors through preprocessing.
• lib/tests/css-auditor.test.mjs -- buildAuditPrompt modern features (STEP 14 + feature id documentation) and CssAuditor modernFeatures integration (parse, empty, missing) blocks.

How to test:

npm test

[nujovich]

All milestones complete. Ready for review when you are.