Fixes #4006.

Summary:

• route CLI progress and log output to stderr when --stdout is used
• keep generated CSS as the only stdout payload
• add a regression test for --stdout output separation

Testing:

• pnpm build
• pnpm typecheck
• pnpm exec vitest packages-engine/cli/test/cli.test.ts --run
• pnpm exec eslint packages-engine/cli/src/index.ts packages-engine/cli/test/cli.test.ts
• pnpm --filter @unocss/cli run build
• git diff --check