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/context-menu/ContextMenuCheckboxItem.vue

@@ -0,0 +1,21 @@
+<script lang="ts">
+import type { MenuCheckboxItemEmits, MenuCheckboxItemProps } from '../menu';
+
+export interface ContextMenuCheckboxItemProps extends MenuCheckboxItemProps {}
+export type ContextMenuCheckboxItemEmits = MenuCheckboxItemEmits;
+</script>
+
+<script setup lang="ts">
+import { MenuCheckboxItem } from '../menu';
+
+const props = defineProps<ContextMenuCheckboxItemProps>();
+const emit = defineEmits<ContextMenuCheckboxItemEmits>();
+</script>
+
+<template>
+  <MenuCheckboxItem
+    v-bind="props"
+    @select="emit('select', $event)"
+    @update:checked="emit('update:checked', $event)"
+  ><slot /></MenuCheckboxItem>
+</template>