refactor: update build config for @primer/react and rename import CSS plugin for Rolldown

[joshblack]

Updated build tooling for @primer/react to use Rolldown, including renaming the internal CSS import plugin package to match Rolldown naming.

Changelog

New

N/A

Changed

• Updated @primer/react to build JavaScript with Rolldown.
• Renamed rollup-plugin-import-css to rolldown-plugin-import-css.
• Updated the import CSS plugin package to export its TypeScript source directly for Rolldown consumption.
• Updated the export size helper and Dependabot bundler grouping for Rolldown.

Removed

• Removed the package Rollup build config and direct Rollup build dependencies.
• Removed the build step and Rollup build config for rolldown-plugin-import-css.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; build tooling-only change with no public runtime API change

Testing & Reviewing

Validated with npm run build, npm run type-check, npm run lint, npm run lint:css, npm run format:diff, and npm test -- --run.

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]

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Latest commit: 39f1f05

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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. If this doesn't work, you can also use the original workflow here. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

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[Copilot]

Pull request overview

This PR refactors @primer/react build tooling to use Rolldown instead of Rollup, and renames the internal CSS import plugin package to align with the Rolldown ecosystem.

Changes:

• Switch @primer/react bundling from Rollup to Rolldown (rolldown.config.mjs, script/build, dependency updates).
• Rename and re-home the internal CSS import plugin as rolldown-plugin-import-css, removing the old Rollup-built package.
• Update CI/supporting tooling (export size script, Dependabot grouping, ESLint paths, lockfile/workspaces) for the Rolldown migration.

Show a summary per file

File	Description
packages/rollup-plugin-import-css/rollup.config.js	Removes obsolete Rollup build config for the old CSS import plugin package.
packages/rollup-plugin-import-css/package.json	Removes the old Rollup plugin package manifest and build scripts/deps.
packages/rolldown-plugin-import-css/tsconfig.json	Adds TS config for the renamed/internal Rolldown CSS import plugin.
packages/rolldown-plugin-import-css/tsconfig.build.json	Adds build TS config (declarations-focused) for the new plugin package.
packages/rolldown-plugin-import-css/src/index.ts	Updates plugin typing import to rolldown.
packages/rolldown-plugin-import-css/README.md	Renames documentation references from Rollup to Rolldown.
packages/rolldown-plugin-import-css/package.json	Introduces new package manifest exporting TS source directly.
packages/react/script/get-export-sizes.cjs	Migrates export-size analysis bundling from Rollup to Rolldown.
packages/react/script/build	Switches bundling invocation from rollup -c to rolldown -c.
packages/react/rolldown.config.mjs	Updates build config to use Rolldown APIs/plugins and preserves directives.
packages/react/package.json	Removes Rollup deps and adds Rolldown + renamed CSS plugin dependency.
package.json	Updates workspace list for the renamed plugin package and removes Rollup optional dep.
package-lock.json	Updates lockfile for Rolldown deps and workspace rename; removes Rollup packages.
eslint.config.mjs	Updates ESLint file globs to target the renamed plugin package directory.
.github/dependabot.yml	Renames Dependabot group from rollup→rolldown and adjusts patterns.

Copilot's findings

Comments suppressed due to low confidence (1)

packages/react/script/get-export-sizes.cjs:92

• The re-export Rolldown build also needs the JSON loader plugin so exports like ./generated/components.json can be bundled/minified for size analysis.

        input: '__entrypoint__',
        external,
        plugins: [
          noopCSSModules,
          virtual({
            __entrypoint__: `export { ${identifier} } from '${filepath}';`,
          }),
        ],
        onwarn: () => {},

• Files reviewed: 12/15 changed files
• Comments generated: 3