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/navigation-menu/NavigationMenuContent.vue

@@ -0,0 +1,72 @@
+<script lang="ts">
+import type { NavigationMenuContentImplEmits, NavigationMenuContentImplProps } from './NavigationMenuContentImpl.vue';
+
+export interface NavigationMenuContentProps extends NavigationMenuContentImplProps {
+  /** Keep mounted regardless of `present`. Useful for transition libraries. */
+  forceMount?: boolean;
+}
+
+export type NavigationMenuContentEmits = NavigationMenuContentImplEmits;
+</script>
+
+<script setup lang="ts">
+import { computed, ref, watch } from 'vue';
+
+import { Presence } from '../presence';
+import { useNavigationMenuContext, useNavigationMenuItemContext } from './context';
+import NavigationMenuContentImpl from './NavigationMenuContentImpl.vue';
+
+defineOptions({ inheritAttrs: false });
+
+const { forceMount = false, ...rest } = defineProps<NavigationMenuContentProps>();
+void rest;
+
+const emit = defineEmits<NavigationMenuContentEmits>();
+
+const menuContext = useNavigationMenuContext();
+const itemContext = useNavigationMenuItemContext();
+
+const open = computed(() => itemContext.value === menuContext.modelValue.value);
+
+// Keep content mounted briefly during viewport animation: if this item was the
+// previously active one we keep present=true until model changes again.
+const isLastActiveValue = ref(false);
+watch(
+  () => menuContext.modelValue.value,
+  (next, prev) => {
+    if (prev === itemContext.value && next !== itemContext.value) isLastActiveValue.value = true;
+    if (next === itemContext.value) isLastActiveValue.value = false;
+  },
+);
+
+const present = computed(() => open.value || isLastActiveValue.value);
+
+function handlePointerEnter() {
+  menuContext.onContentEnter(itemContext.value);
+  emit('pointerEnterContent');
+}
+
+function handlePointerLeave() {
+  menuContext.onContentLeave();
+  emit('pointerLeaveContent');
+}
+</script>
+
+<template>
+  <Teleport :to="menuContext.viewport.value ?? 'body'" :disabled="!menuContext.viewport.value">
+    <Presence :present="present" :force-mount="forceMount || !menuContext.unmountOnHide.value">
+      <NavigationMenuContentImpl
+        v-bind="$attrs"
+        @escape-key-down="emit('escapeKeyDown', $event)"
+        @pointer-down-outside="emit('pointerDownOutside', $event)"
+        @focus-outside="emit('focusOutside', $event)"
+        @interact-outside="emit('interactOutside', $event)"
+        @dismiss="emit('dismiss')"
+        @pointerenter="handlePointerEnter"
+        @pointerleave="handlePointerLeave"
+      >
+        <slot />
+      </NavigationMenuContentImpl>
+    </Presence>
+  </Teleport>
+</template>