refactor(list): rename item group header and list mixin

[rdlabo]

Summary

• rename .header-item-group to .item-group-header across the demo and theme selector
• rename utils/theme-list-inset.scss to utils/structured-list.scss
• expose the internal mixin as structured-list.layout($outer-note-inset)
• add an English migration guide with a copyable template diff
• keep the repository guide out of the portal until it is registered there

Validation

• npm run build
• Prettier via lint-staged
• verified that old class, module, and mixin names no longer remain in theme/demo sources
• independent OSS documentation review approved

[github-actions]

Playwright test results

 78 passed

Details

 78 tests across 1 suite
 1 minute, 28 seconds
 97204b5
 This detailed result covers Ionic 9 only. Ionic 8 runs against the same screenshots in a separate matrix job; check the workflow run for both results. To update the screenshots, comment with /update-screenshots.

[github-actions]

📊 Ionic 9 Playwright Test Report

View the detailed Ionic 9 report: https://rdlabo-dev.github.io/ionic-theme-md3/pr-21/

Ionic 8 runs against the same screenshots in a separate matrix job. View both results in the workflow run.

[github-actions]

📊 Ionic 9 Playwright Test Report

View the detailed Ionic 9 report: https://rdlabo-dev.github.io/ionic-theme-md3/pr-21/

Ionic 8 runs against the same screenshots in a separate matrix job. View both results in the workflow run.

[github-actions]

📊 Ionic 9 Playwright Test Report

View the detailed Ionic 9 report: https://rdlabo-dev.github.io/ionic-theme-md3/pr-21/

Ionic 8 runs against the same screenshots in a separate matrix job. View both results in the workflow run.