feat(macos): support applicationDockMenu: (right-click Dock menu)

[thomnico]

Summary

Adds support for applicationDockMenu: — the menu Cocoa shows when the user right-clicks (or Control-clicks / long-presses) the application's Dock icon. Currently unsupported: macOS falls back to its default minimal menu (Show/Hide, Quit) with no way for the app to customize it.

New public API on EventLoopWindowTargetExtMacOS:

fn set_dock_menu(&self, menu: Option<objc2::rc::Retained<objc2_app_kit::NSMenu>>);

Same shape as the existing set_badge_label/set_dock_visibility methods. Can be called at any point after the event loop starts, including from a menu item's own click handler (e.g. to rebuild the menu with an updated checkmark) — the next right-click picks up the change.

Why a small module instead of an Event variant

applicationDockMenu: must return its value synchronously to Cocoa — there's no round-trip available through tao's event queue the way there is for e.g. Event::Reopen. So the menu is stored in a small module-level Mutex (platform_impl/macos/dock_menu.rs), read directly by the new application_dock_menu handler in app_delegate.rs. Same main-thread-only invariant as the rest of that file's id usage (documented inline).

Testing

Used this in a real app (menu bar app built on tao 0.36, EventLoopBuilder::<UserEvent>::with_user_event()) — right-click on the Dock icon now shows a custom CheckMenuItem built with muda, synced with the same toggle exposed elsewhere in the app (tray menu, application menu bar). Confirmed working on macOS (tested by the end user, not just compiled).

Notes

• Added "NSMenu" to the objc2-app-kit feature list in Cargo.toml (was missing, needed for NSMenu/Retained<NSMenu>).
• cargo fmt --check and cargo check --target aarch64-apple-darwin pass.
• Happy to add a dock_menu example under examples/ if useful for review — didn't want to guess at what you'd want there without asking first.