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/MenuCheckboxItem.vue

@@ -0,0 +1,57 @@
+<script lang="ts">
+import type { MenuItemImplEmits, MenuItemImplProps } from './MenuItemImpl.vue';
+import type { CheckedState } from './types';
+
+export interface MenuCheckboxItemProps extends MenuItemImplProps {
+  checked?: CheckedState;
+  defaultChecked?: CheckedState;
+}
+export interface MenuCheckboxItemEmits extends MenuItemImplEmits {
+  'update:checked': [value: CheckedState];
+}
+</script>
+
+<script setup lang="ts">
+import { computed, ref } from 'vue';
+
+import { provideMenuItemIndicatorContext, useMenuRootContext } from './context';
+import MenuItemImpl from './MenuItemImpl.vue';
+import { ITEM_SELECT, getCheckedState, isIndeterminate } from './utils';
+
+const {
+  checked: checkedProp,
+  defaultChecked = false,
+  ...itemProps
+} = defineProps<MenuCheckboxItemProps>();
+
+const emit = defineEmits<MenuCheckboxItemEmits>();
+const rootCtx = useMenuRootContext();
+
+const local = ref<CheckedState>(defaultChecked);
+const checkedState = computed<CheckedState>(() => checkedProp !== undefined ? checkedProp : local.value);
+
+provideMenuItemIndicatorContext({ checkedState });
+
+function handleSelect(event: Event) {
+  const next: CheckedState = isIndeterminate(checkedState.value) ? true : !checkedState.value;
+  local.value = next;
+  emit('update:checked', next);
+
+  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="itemProps"
+    role="menuitemcheckbox"
+    :aria-checked="isIndeterminate(checkedState) ? 'mixed' : checkedState"
+    :data-state="getCheckedState(checkedState)"
+    @select="handleSelect"
+  >
+    <slot />
+  </MenuItemImpl>
+</template>