Highlight color improvements for lists

[jondkinney]

Summary

• Clear highlight color on Enter — new paragraphs start with default styling
• Parent retention: children of uniformly highlighted list items inherit color
• Bullet marker color sync via ListItemNode transform (::before matches text)
• Highlight propagation: applying color to parent cascades to nested children
• Mark padding sync: data-pad-start/data-pad-end on <mark> elements
• CSS parsing helpers that handle var() values safely in Rollup builds
• All inheritance functions exported for reuse by other extensions

Test plan

• Apply highlight color, press Enter — new paragraph has no color
• Highlight a parent list item, Tab-indent a child — child inherits color
• Colored text bullet markers (::before) match text color
• Apply color to parent with nested children — all children update

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