add CSS migration foundation and screenshot tooling (Phase 0)
- CSS file structure with @layer declaration (reset, layout, components, utilities, overrides)
- Layout primitives: .stack, .cluster, .row, .auto-grid, .container-page, .with-sidebar, .center
- mix screenshots task using Playwright for visual regression testing
- Golden baseline captured (10 pages x 4 breakpoints = 40 screenshots)
- No visual changes — new CSS not wired into any layout yet
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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