E3.1.3 Add --ground-tolerance #55

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

Task

Add --ground-tolerance

Subtasks

• Default stays 3.0
• Document when to raise it

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