refactor: simplify inlineCss conditional logic and remove dead code
- Restructure render-css-resource.tsx to use clearer early returns
- Extract isRSCRequest to local variable for readability
- Remove unreachable fallback <style> render in 'shared' mode
- Initialize rootLayoutCSSPaths at creation instead of lazily
- Make rootLayoutCSSPaths required in CollectedInlineCss type
- Improve JSDoc to explain CSS duplication with inlineCss: true