refactor(css): replacing slow selector

[mfranzke]

Proposed changes

Replacing a CSS selector (:is(:root, :host)) with a simpler one (:root, :host) that consistently appeared in the CSS Selectors stats as the lowest performer (and is the most low-hanging fruit of all).

Types of changes

• Bugfix (non-breaking change that fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improvements to existing components or architectural decisions)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Further comments

🔭🐙🐈 Test this branch here: https://design-system.deutschebahn.com/core-web/review/refactor-css-replacing-slow-selector

[changeset-bot]

🦋 Changeset detected

Latest commit: 3d5cb46

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 9 packages

Name	Type
@db-ux/core-foundations	Patch
@db-ux/core-components	Patch
@db-ux/ngx-core-components	Patch
@db-ux/react-core-components	Patch
@db-ux/wc-core-components	Patch
@db-ux/v-core-components	Patch
@db-ux/core-stylelint	Patch
@db-ux/core-migration	Patch
@db-ux/agent-cli	Patch

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[Copilot]

Pull request overview

This PR optimizes CSS selector performance by replacing the slower :is(:root, :host) pseudo-class syntax with the equivalent but faster :root, :host multi-selector syntax across the foundations package.

• Replaces :is(:root, :host) with :root, :host in 5 SCSS files
• Maintains identical functionality and specificity
• Addresses performance bottleneck identified in CSS Selectors stats

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/foundations/scss/defaults/default-root.scss	Updated root/host selector for default styles
packages/foundations/scss/defaults/default-required.scss	Updated root/host selector in variables layer
packages/foundations/scss/defaults/_default-variables.scss	Updated root/host selector for CSS custom properties
packages/foundations/scss/defaults/_default-container-variables.scss	Updated root/host selector for container variables
packages/foundations/scss/colors/_default-color-mappings.scss	Updated root/host selector in color mappings
.changeset/rich-baboons-compete.md	Added changeset documenting the refactor across all affected packages