fix: show subtitles on submenus

[juliusmarminge]

Overview

iOS currently applies subtitle only when an action becomes a leaf UIAction. An action with subactions becomes a UIMenu, so its subtitle is silently dropped.

This assigns UIMenu.subtitle on iOS 15+ and derives a stable UIMenu.Identifier from the action id for navigable submenus. Stable identifiers let UIKit match submenu nodes during in-place menu updates without returning the user to the root menu. Inline sections do not navigate, so they keep UIKit's stock identifier behavior.

Related to #536, which reported missing iOS subtitles. Leaf action subtitles are supported today, but the submenu path remained unhandled.

Companion to #1220, which refreshes presented menus in place and relies on these stable identifiers to preserve the current submenu level.

Test Plan

• yarn lint
• yarn typescript
• yarn test --runInBand
• Generated the new-architecture iOS example workspace with CocoaPods/codegen
• Built the iOS example for a generic simulator with the new architecture enabled

Generated with OpenAI Codex (GPT-5) in T3 Code.