feat: add feature plugin tests and validation for feature flags

vite-layers/example/apps/aurora/app.config.ts

@@ -0,0 +1,10 @@
+import { defineLayerConfig } from '../../../src/index.ts'
+
+// Aurora — a dark-themed brand. Like Northwind it inherits the whole shell from `main` and changes
+// only its logo, theme.css and Landing. It keeps the billing page but turns off the beta accent,
+// showing that each brand toggles a different subset of features.
+export default defineLayerConfig({
+  name: 'aurora',
+  extends: ['../main'],
+  features: { betaBanner: false }, // keeps `billing` (inherited true); drops the beta pill
+})