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/MenuArrow.vue

@@ -0,0 +1,17 @@
+<script lang="ts">
+import type { PopperArrowProps } from '../popper';
+
+export interface MenuArrowProps extends PopperArrowProps {}
+</script>
+
+<script setup lang="ts">
+import { PopperArrow } from '../popper';
+
+const props = defineProps<MenuArrowProps>();
+</script>
+
+<template>
+  <PopperArrow v-bind="props">
+    <slot />
+  </PopperArrow>
+</template>