docs(Heading): explain when to use roles or themes - #6081

Summary

• explain when to use a custom Heading visual role versus a separate theme
• document level, type, and weight responsibilities and runtime theme ownership
• add the decision guidance to the accepted spec, Heading consumer docs, and starter theme template

Stack

• depends on feat(core): add Heading weight override #6027 , which depends on feat(core): support custom Heading visual types #6026
• documentation-only diff relative to feat(core): add Heading weight override #6027

Validation

• pnpm check:knowledge
• pnpm check:cli-structure
• pnpm check:changesets
• Prettier check
• targeted ESLint (no errors; Heading doc is ignored by the repository ESLint configuration)
• git diff --check