DropdownMenu: nested submenus (flyouts)

[czarandy]

Feature

DropdownMenu supports only a flat list with one-level non-interactive sections — DropdownMenuOption is Divider | ItemData | Section, and ItemData has no recursion (src/components/DropdownMenu/types.ts); navigation is linear useListFocus (src/components/DropdownMenu/menuUtils.tsx).

Add nested submenus:

• Data API: allow items on an item (nested array) to declare a submenu; optionally a composable sub-menu component for the JSX API.
• Flyout opens toward inline-end with auto-flip when there's no room; RTL-aware.
• Hover-intent (small delay / diagonal tolerance) so the submenu doesn't flicker while traversing.
• Full keyboard support: inline-end arrow opens and focuses first item, inline-start arrow closes back to the parent, Enter/Space activate; parent item gets aria-haspopup="menu" + aria-expanded.

Notes

• ContextMenu shares the item/keyboard infrastructure and should get submenus for free (or explicitly out of scope — decide in the PR).
• Tests: pointer open/close, hover-intent, keyboard traversal, RTL direction.
• Storybook example with two levels.