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/MenuRoot.vue

@@ -0,0 +1,57 @@
+<script lang="ts">
+import type { Direction } from '../config-provider';
+
+export interface MenuRootProps {
+  open?: boolean;
+  dir?: Direction;
+  modal?: boolean;
+}
+export interface MenuRootEmits {
+  'update:open': [value: boolean];
+}
+</script>
+
+<script setup lang="ts">
+import { shallowRef, toRef } from 'vue';
+
+import { useConfig } from '../config-provider';
+import { PopperRoot } from '../popper';
+import { provideMenuContext, provideMenuRootContext } from './context';
+import { useIsUsingKeyboard } from './useIsUsingKeyboard';
+
+const {
+  open = false,
+  dir: dirProp,
+  modal = true,
+} = defineProps<MenuRootProps>();
+
+const emit = defineEmits<MenuRootEmits>();
+
+defineSlots<{ default?: () => unknown }>();
+
+const config = useConfig();
+const dirRef = toRef(() => dirProp ?? config.dir.value);
+const isUsingKeyboardRef = useIsUsingKeyboard();
+const content = shallowRef<HTMLElement | null>(null);
+const openRef = toRef(() => open);
+
+provideMenuContext({
+  open: openRef,
+  onOpenChange: v => emit('update:open', v),
+  content,
+  onContentChange: (el) => { content.value = el; },
+});
+
+provideMenuRootContext({
+  onClose: () => emit('update:open', false),
+  dir: dirRef,
+  isUsingKeyboardRef,
+  modal: toRef(() => modal),
+});
+</script>
+
+<template>
+  <PopperRoot>
+    <slot />
+  </PopperRoot>
+</template>