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/NavigationMenuItem.vue

@@ -0,0 +1,80 @@
+<script lang="ts">
+export interface NavigationMenuItemProps {
+  /**
+   * Unique value associating this item with the active state. Generated
+   * automatically when omitted.
+   */
+  value?: string;
+}
+</script>
+
+<script setup lang="ts">
+import { computed, ref, shallowRef, toValue } from 'vue';
+
+import { focusFirst, getTabbableCandidates } from '@robonen/platform/browsers';
+import { useForwardExpose, useId } from '@robonen/vue';
+import { Primitive } from '../primitive';
+import { provideNavigationMenuItemContext, useNavigationMenuContext } from './context';
+import { makeContentId, makeTriggerId, removeFromTabOrder } from './utils';
+
+const { value: valueProp } = defineProps<NavigationMenuItemProps>();
+
+useForwardExpose();
+
+const context = useNavigationMenuContext();
+
+const autoId = useId(undefined, 'primitives-navigation-menu-item');
+const value = computed<string>(() => valueProp ?? autoId.value);
+
+const triggerRef = shallowRef<HTMLElement | undefined>(undefined);
+const focusProxyRef = shallowRef<HTMLElement | undefined>(undefined);
+const wasEscapeCloseRef = ref(false);
+
+const triggerId = computed(() => makeTriggerId(toValue(context.baseId), value.value));
+const contentId = computed(() => makeContentId(toValue(context.baseId), value.value));
+
+let restoreContentTabOrder: () => void = () => {};
+
+function handleContentEntry(side: 'start' | 'end' = 'start') {
+  const el = document.getElementById(contentId.value);
+  if (!el) return;
+  restoreContentTabOrder();
+  const candidates = getTabbableCandidates(el);
+  if (candidates.length) {
+    focusFirst(side === 'start' ? candidates : [...candidates].reverse());
+  }
+}
+
+function handleContentExit() {
+  const el = document.getElementById(contentId.value);
+  if (!el) return;
+  const candidates = getTabbableCandidates(el);
+  if (candidates.length) {
+    restoreContentTabOrder = removeFromTabOrder(candidates);
+  }
+}
+
+provideNavigationMenuItemContext({
+  get value() { return value.value; },
+  get contentId() { return contentId.value; },
+  get triggerId() { return triggerId.value; },
+  triggerRef,
+  onTriggerChange: (el) => { triggerRef.value = el; },
+  focusProxyRef,
+  onFocusProxyChange: (el) => { focusProxyRef.value = el; },
+  wasEscapeCloseRef,
+  onEntryKeyDown: () => handleContentEntry('start'),
+  onFocusProxyEnter: side => handleContentEntry(side),
+  onContentFocusOutside: handleContentExit,
+  onRootContentClose: handleContentExit,
+});
+</script>
+
+<template>
+  <Primitive
+    as="li"
+    data-primitives-navigation-menu-item
+  >
+    <slot />
+  </Primitive>
+</template>