Add CSS style authoring support

[pcharbon70]

Summary

• add the CSS style authoring ADR, specs, and phased traceability plan
• add a css block to the UnifiedUi DSL, parsed through a csserpent-backed adapter
• implement selector matching, cascade resolution, declaration translation, diagnostics, and IUR style provenance
• document runtime boundaries and add LiveUi renderer coverage for CSS-derived canonical styles

Stack

• Base branch: codex/canonical-widget-components
• Head branch: codex/css-style-authoring

Verification

• mix test in packages/unified-ui
• mix test test/live_ui/renderer_style_test.exs in packages/live_ui
• mix spec.verify --debug
• mix spec.check
• mix spec.diffcheck
• git diff --check