build: align Vite 8 bundler config

[southpolesteve]

Summary

• bump the workspace catalog and vinext peer range to the current Vite 8 plugin-rsc/plugin-react releases
• emit rolldownOptions on Vite 8 while still honoring user-supplied legacy build.rollupOptions
• switch client chunking to Rolldown codeSplitting on Vite 8 and update tests to accept both config shapes

Testing

• pnpm test tests/build-optimization.test.ts
• pnpm test tests/pages-router.test.ts -t "suppresses MODULE_LEVEL_DIRECTIVE warnings from Rollup|preserves user-supplied build.rollupOptions.onwarn"
• pnpm test tests/tsconfig-paths-vite8.test.ts
• pnpm run build