menu: fix submenu dismiss propagation and ESC handling (#2232)

## Summary

Fixes #2229 — three related fixes for submenu dismiss behavior in
`PopupMenu`:

1. **Auto-wire `parent_menu` in `render()`**: Submenus added via
`PopupMenuItem::submenu()` (the data constructor) don't get
`parent_menu` set, unlike `PopupMenu::submenu()` (the builder method).
This breaks the dismiss chain. Now `PopupMenu::render()` auto-wires
`parent_menu` on any submenu child that doesn't have it set. This is
needed because table delegates operate in `Context<TableState>` and
cannot call `PopupMenu::submenu()` which requires `Context<PopupMenu>`.

2. **Fix ESC not closing menus when a submenu is open**: When a submenu
is opened by hover, the parent menu retains focus. Pressing ESC would
hit the parent's `dismiss()`, which bailed out early because
`active_submenu()` was `Some`. Split `dismiss` into `dismiss` (ESC
action handler that clears the active submenu first) and `dismiss_menu`
(internal dismiss that propagates up the chain).

3. **Guard `handle_dismiss` for submenu clicks**: `handle_dismiss`
(click-outside) returns early if a submenu is active. Without this,
clicking a submenu item triggers the parent's `on_mouse_down_out`,
tearing down the submenu before the item's `on_click` fires.

Changes are in `popup_menu.rs` only — no table code changes needed.

No dependency on #2231.

## AI Disclosure

Implementation was developed with AI assistance (Claude). All code has
been reviewed, tested, and follows existing `popup_menu.rs` patterns.

## Test plan

- [x] `cargo run --example table` — right-click context menu with
submenus works
- [x] `cargo run` (story) — Menu story context menu: ESC closes full
chain
- [x] Submenu item click fires handler and dismisses menu
- [x] Click outside closes all menus
- [x] Hover navigation between submenus still works

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Co-authored-by: Jason Lee <huacnlee@gmail.com>