Fix PostCSS packaging and fixture compatibility

[Austin1serb]

Summary

• stop the PostCSS runtime from invoking CLI init during builds and lazily load its ESM helpers so the CJS entry is require-safe
• update useScopedUI to expose a stable callback ref compatible with newer React typings
• refresh the Next and Vite fixtures to current dependency stacks and align brittle Next color assertions with Chromium's reported lab(...) values

Verification

• pnpm test
• pnpm --dir packages/core run smoke
• manual fixture builds for upgraded Next and Vite test apps

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Project	Deployment	Actions	Updated (UTC)
react-zero-ui	Error		Apr 6, 2026 6:15am

[Austin1serb]

Looks good