Report CSS size stats in CI size summary

[Copilot]

Updates the CI size report for @primer/react to include CSS imported by each entrypoint/export, plus cssstats metrics for quick review of CSS footprint.

Changelog

New

• Adds CSS reporting columns to the GitHub Actions size summary:
  • CSS gzip size
  • CSS size
  • CSS rules
  • CSS selectors
  • CSS declarations
  • max selector specificity
• Adds cssstats as a dev dependency for CI reporting.

Changed

• Updates packages/react/script/get-export-sizes.cjs to collect CSS modules during Rollup analysis and associate included CSS with each reported entrypoint/export.

Removed

• Nothing.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; CI/reporting-only change.

Testing & Reviewing

Review the generated Actions summary table for the new CSS columns and cssstats values.

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]

⚠️ No Changeset found

Latest commit: 0c56f9e

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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