css: consolidate core stylesheet entrypoint and add contract check

README.md

@@ -200,11 +200,13 @@ Zentrale Partials:
 
 ## Asset- und Frontend-System
 
-- CSS-Layer-Entrypoint: `/web/css/app-layers.css`
+- Core-CSS-Entrypoint: `/web/css/core.css`
+- CSS-Layer/Vendor-Basis: `/web/css/app-layers.css`
 - Vendor-Overrides: `/web/css/vendor-overrides`
 - Asset-Gruppen: `/config/assets.php`
 - Template-Styles: `renderTemplateStyles('default'|'login')`
 - Seiten-Styles per Action: `Buffer::set('style_groups', ...)`
+- CSS-Contract-Check: `bin/css-contract-check.sh`
 - JS-Bootstrap:
   - `/web/js/app-boot.js` (frueh, ohne Module)
   - `/web/js/app-init.js` (Standardseiten)