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/menubar/MenubarContent.vue

@@ -0,0 +1,46 @@
+<script lang="ts">
+import type { MenuContentEmits, MenuContentProps } from '../menu';
+
+export interface MenubarContentProps extends MenuContentProps {}
+export type MenubarContentEmits = MenuContentEmits;
+</script>
+
+<script setup lang="ts">
+import { MenuContent } from '../menu';
+import { useMenubarMenuContext, useMenubarRootContext } from './context';
+
+const props = defineProps<MenubarContentProps>();
+const emit = defineEmits<MenubarContentEmits>();
+
+const rootCtx = useMenubarRootContext();
+const menuCtx = useMenubarMenuContext();
+</script>
+
+<template>
+  <MenuContent
+    :id="menuCtx.contentId.value"
+    v-bind="props"
+    align="start"
+    :aria-labelledby="menuCtx.triggerId.value"
+    @close-auto-focus="(event: Event) => {
+      if (!menuCtx.wasKeyboardTriggerOpenRef.value) event.preventDefault()
+      menuCtx.wasKeyboardTriggerOpenRef.value = false
+      menuCtx.triggerRef.value?.focus({ preventScroll: true })
+      emit('closeAutoFocus', event)
+    }"
+    @escape-key-down="emit('escapeKeyDown', $event)"
+    @pointer-down-outside="(event: PointerEvent | MouseEvent) => {
+      const target = event.target as Node
+      const isMenubarTrigger = menuCtx.triggerRef.value?.contains(target)
+      if (isMenubarTrigger) event.preventDefault()
+      emit('pointerDownOutside', event)
+    }"
+    @focus-outside="emit('focusOutside', $event)"
+    @interact-outside="emit('interactOutside', $event)"
+    @dismiss="rootCtx.onMenuClose()"
+    @entry-focus="emit('entryFocus', $event)"
+    @open-auto-focus="emit('openAutoFocus', $event)"
+  >
+    <slot />
+  </MenuContent>
+</template>