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/MenuSubTrigger.vue

@@ -8,7 +8,7 @@ export interface MenuSubTriggerProps extends MenuItemImplProps {}
 import { PopperAnchor } from '../popper';
 import { useMenuContentContext, useMenuContext, useMenuRootContext, useMenuSubContext } from './context';
 import MenuItemImpl from './MenuItemImpl.vue';
-import { SUB_CLOSE_KEYS, SUB_OPEN_KEYS, getOpenState, isPointerInGraceArea } from './utils';
+import { SUB_CLOSE_KEYS, SUB_OPEN_KEYS, getOpenState } from './utils';
 
 const props = defineProps<MenuSubTriggerProps>();
 
@@ -62,7 +62,7 @@ function handleKeyDown(event: KeyboardEvent) {
 </script>
 
 <template>
-  <PopperAnchor as-child>
+  <PopperAnchor>
     <MenuItemImpl
       v-bind="props"
       :id="subCtx.triggerId.value"