fix(primitives): eslint/tsconfig migration, asChild refactor, type fixes

- Migrate to eslint flat config + composite tsconfig.
- Complete the asChild→as="template" refactor (remove asChild prop + :as-child
  bindings across components, matching Primitive's slot model).
- Fix test type errors and source type-safety (useGraceArea hull/point math,
  FocusScope/util ref typing).

Note: ~53 vue-tsc errors remain (HTML attr/event passthrough typing on
transparent wrapper components + a couple of duplicate-export naming
collisions) — not gated by CI (build/lint/test green); pending a
component-attribute-typing design decision.

vue/primitives/src/menu/MenuRadioGroup.vue

@@ -16,7 +16,7 @@ import { computed, ref } from 'vue';
 import { Primitive } from '../primitive';
 import { provideMenuRadioGroupContext } from './context';
 
-const { modelValue, defaultValue, as = 'div', asChild } = defineProps<MenuRadioGroupProps>();
+const { modelValue, defaultValue, as = 'div' } = defineProps<MenuRadioGroupProps>();
 const emit = defineEmits<MenuRadioGroupEmits>();
 
 const local = ref(defaultValue);
@@ -32,7 +32,7 @@ provideMenuRadioGroupContext({
 </script>
 
 <template>
-  <Primitive :as="as" :as-child="asChild" role="group">
+  <Primitive :as="as" role="group">
     <slot />
   </Primitive>
 </template>