menu: Support async submenu content with rebuild and late parent wiring (#2609)

## Description

This PR makes `PopupMenu` usable for menus whose content arrives
asynchronously after the menu is shown:

- Added `PopupMenu::rebuild` to replace all menu items by re-running a
builder on the existing menu entity, keeping its identity (focus,
`parent_menu`, layer priority). This allows swapping a "loading…"
placeholder for the loaded items once an async fetch completes, without
recreating the menu.
- Submenus attached via the public `item()` + `PopupMenuItem::submenu()`
path (from contexts that only have the menu value, e.g. a table
delegate's `context_menu`) previously had no `parent_menu` or paint
priority wired. They are now wired lazily in `render`, so the dismiss
chain, click-outside checks, and keyboard navigation treat them the same
as `submenu()`-built children.

No breaking changes; both changes are additive.

> Note: Portions of this PR were AI-generated and have been reviewed.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>