feat(primitives): add menu, dropdown-menu, context-menu, and menubar primitives

Implements WAI-ARIA APG-compliant headless menu primitive families:

- menu: base primitive with MenuRoot, MenuContent, MenuItem,
  MenuCheckboxItem, MenuRadioGroup/Item, MenuSub, and helpers
- dropdown-menu: DropdownMenuRoot with trigger anchoring
- context-menu: ContextMenuRoot with right-click virtual anchor
- menubar: MenubarRoot with keyboard navigation between menus

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

vue/primitives/src/menu/MenuItem.vue

@@ -0,0 +1,32 @@
+<script lang="ts">
+import type { MenuItemImplEmits, MenuItemImplProps } from './MenuItemImpl.vue';
+
+export interface MenuItemProps extends MenuItemImplProps {}
+export type MenuItemEmits = MenuItemImplEmits;
+</script>
+
+<script setup lang="ts">
+import { useMenuRootContext } from './context';
+import MenuItemImpl from './MenuItemImpl.vue';
+import { ITEM_SELECT } from './utils';
+
+const props = defineProps<MenuItemProps>();
+const emit = defineEmits<MenuItemEmits>();
+
+const rootCtx = useMenuRootContext();
+
+function handleSelect(event: Event) {
+  const selectEvent = new CustomEvent(ITEM_SELECT, { bubbles: true, cancelable: true })
+  ;(event.currentTarget as HTMLElement).dispatchEvent(selectEvent);
+  emit('select', event);
+  if (!selectEvent.defaultPrevented) {
+    rootCtx.onClose();
+  }
+}
+</script>
+
+<template>
+  <MenuItemImpl v-bind="props" @select="handleSelect">
+    <slot />
+  </MenuItemImpl>
+</template>