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/context-menu/ContextMenuTrigger.vue

@@ -14,7 +14,7 @@ import { MenuAnchor, useMenuContext } from '../menu';
 import { Primitive } from '../primitive';
 import { useContextMenuRootContext } from './context';
 
-const { disabled = false, as = 'span', asChild } = defineProps<ContextMenuTriggerProps>();
+const { disabled = false, as = 'span' } = defineProps<ContextMenuTriggerProps>();
 
 const menuCtx = useMenuContext();
 const ctxMenuCtx = useContextMenuRootContext();
@@ -72,11 +72,10 @@ function handlePointerUp() {
 </script>
 
 <template>
-  <MenuAnchor as-child :reference="virtualEl">
+  <MenuAnchor :reference="virtualEl">
     <Primitive
       :ref="forwardRef"
       :as="as"
-      :as-child="asChild"
       :data-state="menuCtx.open.value ? 'open' : 'closed'"
       :data-disabled="disabled ? '' : undefined"
       @contextmenu="handleContextMenu"