Updates to Radio group

Internal build-tool change with no consumer-facing API differences — dist shape, exports map, and type declarations are unchanged. - Rolldown preserves "use client" directives on entries natively, so the post-build hook that previously re-inserted them is gone. - build:css now invokes the tailwindcss bin directly instead of npx @tailwindcss/cli, eliminating stray npm warnings during builds. - Adds RefAttributes to RadioGroup.Root and RadioGroup.Item prop types so they match the Select/Combobox/Autocomplete convention. - Adds docs/contributing/BASE_UI.md as the canonical authoring guide for new Base UI wrappers.