Propagate CSS-derived styles to runtime renderers

[pcharbon70]

Summary

• translate canonical UnifiedIUR.Style values into native style maps for elm_ui, desktop_ui, and terminal_ui renderers
• extend Elm frontend style realization for canonical foreground and border-color values
• add runtime package tests proving CSS-derived canonical styles render without parsing authored CSS or leaking CSS provenance

Stack

• Base branch: codex/canonical-widget-components
• Depends on merged PR Add CSS style authoring support #152 (codex/css-style-authoring)

Verification

• mix test in packages/elm_ui
• mix test in packages/desktop_ui
• mix test in packages/terminal_ui
• mix spec.diffcheck
• git diff --check