Prepare Emulsify Tools 2.1.0 release

[callinmullaney]

Summary

• Add PHPCS enforcement, raise PHPStan coverage, and add kernel coverage for Twig namespace and admin favicon integrations.
• Add structured Drush favicon output, --all support, persistent Twig namespace registry caching, admin favicon cacheability metadata, and stricter child theme machine-name validation.
• Clean up release/docs hygiene by standardizing on LICENSE, fixing 2.x README references, and enabling maintained CHANGELOG.md generation through semantic-release.
• Chose semantic-release option A because the changelog/git plugins do not conflict with Drupal.org sync: semantic-release creates and pushes the [skip ci] release commit first, then the workflow mirrors HEAD:main and tags to Drupal.org.

Related Issue(s)

• None.

Validation

• npm run lint
• composer test:unit
• composer test:unit -- --testsuite kernel
• composer test:deprecations
• node --check release.config.cjs
• Semantic-release plugin order load check
• git diff --check

Notes

• Branch: release/2.1.0
• This PR prepares the 2.1.0 release branch from the local commits ahead of origin/main.
• Semantic-release now maintains CHANGELOG.md and commits release assets before the existing Drupal.org tag-sync step runs.

Security

• No credential, filesystem permission, or output-sanitization behavior changes.
• The child theme Drush command now rejects invalid, reserved, or colliding machine names earlier with actionable errors.

Accessibility

• No UI or generated markup changes requiring manual accessibility review.

[github-actions]

🎉 This PR is included in version 2.1.0 🎉

The release is available on GitHub release

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