feat: add CSS Layer support to layout foundations

[joshblack]

Part of https://github.com/github/primer/issues/6275

Migrates Link, Overlay, Pagehead, PageHeader, Pagination, Placeholder, Popover, ProgressBar, and Radio styles into named Primer component CSS layers.

Changelog

New

• Adds CSS layer coverage for Link, Overlay, Pagehead, PageHeader, Pagination, Placeholder, Popover, ProgressBar, and Radio.

Changed

• Wraps Link, Overlay, Pagehead, PageHeader, Pagination, Placeholder, Popover, ProgressBar, and Radio CSS module styles in named @layer primer.components.* layers.

Removed

• None

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

• Run cd packages/react && npx vitest run src/__tests__/css-layers.test.ts --config vitest.config.mts.
• Review this PR as part of the CSS layers stack; this entry covers Link, Overlay, Pagehead, PageHeader, Pagination, Placeholder, Popover, ProgressBar, and Radio.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: 7c570b2

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

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

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[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

To publish a canary release for integration testing, apply the Canary Release label to this PR.