E3.1.2 Fix auto ground detection #54

Part of: content icon normalize --ground auto keeps pale grounds opaque on dark-card families and blows the 300 KB budget (E3.1).

Task

Fix auto ground detection

Subtasks

• Key a uniform ground whatever the card colour
• Keep full-bleed for avatar-style tiles with all four corners alike

Definition of done

• Tests added or updated and CI green
• Docs updated when the command surface or output changes
• No private registry data, local paths or private repository names in fixtures or docs