fix(macos): show React menu on right-click of Fabric root views

[Saadnajmi]

Summary

On the new architecture, right-clicking inside the React content of the macOS test app shows nothing. On the old architecture, RCTRootView.menuForEvent: returns the dev menu, but the Fabric root view (RCTSurfaceHostingProxyRootView) does not override menuForEvent:, so the menu never appears.

This adds a small HostingViewController that overrides mouseDown / rightMouseDown to pop up the React menu, and:

• Uses it as the wrapper view controller in AppDelegate.present() (multi-app mode, after a component is presented).
• Ungates the existing handlers on ViewController so they also fire in ENABLE_SINGLE_APP_MODE, where the storyboard-instantiated ViewController is in the responder chain above the React root view.

The popUpReactMenu helper is moved out of the #if !ENABLE_SINGLE_APP_MODE guard so it is available in single-app mode too.

Relationship to microsoft/rnx-kit#4114

This is the self-contained workaround of a pair. The underlying root cause — that consumers of host.viewWithModuleName: bypass RCTRootViewFactory and so the upstream devMenu wiring on RCTSurfaceHostingView (react-native-macos 0.81+) never runs — is fixed properly in microsoft/rnx-kit#4114. That PR makes @rnx-kit/react-native-host set devMenu on 0.81+ and installs a fallback gesture recognizer on older versions, so the upstream React Native dev menu (Reload, Open Debugger, …) shows on secondary-click without the test app needing to do anything.

The two PRs differ in what menu they show:

• This PR shows the test app's own React menu (component picker + Load From Dev Server + …).
• fix(react-native-host): wire dev menu on macOS Fabric root views rnx-kit#4114 shows the standard RN dev menu.

Once microsoft/rnx-kit#4114 lands, is published, and the test app bumps @rnx-kit/react-native-host, this workaround can be removed in favor of the upstream behavior — or kept if we prefer the test app's richer React menu over the bare RN dev menu. They will conflict if both are active, since the rnx-kit gesture recognizer fires before the responder chain reaches HostingViewController, so this should be removed at the same commit that picks up the new rnx-kit version.

Test plan

• Right-click the React content on macOS, new architecture, single-app mode → React menu pops up.
• Right-click the React content on macOS, new architecture, multi-app mode (after a component is presented) → React menu pops up.
• Empty initial state in multi-app mode still shows the React menu on click as before.
• Old architecture continues to work unchanged.

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