Add support for nested menus to `@daffodil/design/menu`

[xelaint]

Feature description

Add support for nested menus to @daffodil/design/menu. Today a <daff-menu> renders a flat list of daff-menu-items. This feature would let a daff-menu-item act as a parent that reveals a child <daff-menu> as a flyout panel, enabling hierarchical menu structures.

Proposed shape — a menu item activates a nested menu much like the existing activator/template pattern:

<ng-template #actionsMenu>
  <daff-menu>
    <button daff-menu-item>Edit</button>

    <!-- parent item that opens a submenu -->
    <button daff-menu-item [daffMenuActivator]="moreMenu">
      More
    </button>

    <a href="/settings" daff-menu-item>Settings</a>
  </daff-menu>
</ng-template>

<ng-template #moreMenu>
  <daff-menu>
    <button daff-menu-item>Duplicate</button>
    <button daff-menu-item>Archive</button>
  </daff-menu>
</ng-template>

Behavior to cover:

• A parent item indicates it has a submenu (e.g. a trailing chevron / daffSuffix) and exposes aria-haspopup="menu" + aria-expanded.
• The child menu is positioned as a flyout adjacent to its parent item, with the existing overlay positioning handling collision/repositioning.
• Opening/closing follows the Menu and Menubar WAI-ARIA pattern the menu already targets, including:
• Right Arrow on a parent item opens the submenu and focuses its first item.
• Left Arrow / Escape closes the submenu and returns focus to the parent item.
• Closing a parent menu closes its open submenus.
• Submenus should nest to arbitrary depth.

Use case

As a developer, I'd like to be able to group secondary actions under a parent menu item so that long or categorized action lists can be organized hierarchically without overflowing a single flat menu.

Prior work

No response

Daffodil version

0.93.0

Additional context

No response