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/MenuRadioGroup.vue

@@ -0,0 +1,38 @@
+<script lang="ts">
+import type { PrimitiveProps } from '../primitive';
+
+export interface MenuRadioGroupProps extends PrimitiveProps {
+  modelValue?: string;
+  defaultValue?: string;
+}
+export interface MenuRadioGroupEmits {
+  'update:modelValue': [value: string];
+}
+</script>
+
+<script setup lang="ts">
+import { computed, ref } from 'vue';
+
+import { Primitive } from '../primitive';
+import { provideMenuRadioGroupContext } from './context';
+
+const { modelValue, defaultValue, as = 'div', asChild } = defineProps<MenuRadioGroupProps>();
+const emit = defineEmits<MenuRadioGroupEmits>();
+
+const local = ref(defaultValue);
+const value = computed(() => modelValue !== undefined ? modelValue : local.value);
+
+provideMenuRadioGroupContext({
+  modelValue: value,
+  onValueChange: (v) => {
+    local.value = v;
+    emit('update:modelValue', v);
+  },
+});
+</script>
+
+<template>
+  <Primitive :as="as" :as-child="asChild" role="group">
+    <slot />
+  </Primitive>
+</template>