feat: add feature plugin tests and validation for feature flags

vite-layers/test/fixtures/devtools/app/app.config.ts

@@ -0,0 +1,5 @@
+export default {
+  name: 'app',
+  extends: ['../base'],
+  features: { billing: false }, // overrides base — disabled leaf → DCE
+}

vite-layers/test/fixtures/devtools/app/public/logo.svg

@@ -0,0 +1 @@
+<svg><!-- app logo --></svg>

vite-layers/test/fixtures/devtools/app/src/components/Header.vue

@@ -0,0 +1 @@
+<template><header>app header</header></template>