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/MenuSeparator.vue

@@ -0,0 +1,17 @@
+<script lang="ts">
+import type { PrimitiveProps } from '../primitive';
+
+export interface MenuSeparatorProps extends PrimitiveProps {}
+</script>
+
+<script setup lang="ts">
+import { Primitive } from '../primitive';
+
+const { as = 'div', asChild } = defineProps<MenuSeparatorProps>();
+</script>
+
+<template>
+  <Primitive :as="as" :as-child="asChild" role="separator" aria-orientation="horizontal">
+    <slot />
+  </Primitive>
+</template>