fix(react-native-host): wire dev menu on macOS Fabric root views

[Saadnajmi]

Summary

On macOS new architecture, right-clicking the React content of an app that uses host.viewWithModuleName:initialProperties: does nothing — the dev menu never appears. The reasons:

1. On react-native-macos 0.81+, RCTSurfaceHostingView does override menuForEvent:, but it returns [_devMenu menu] and _devMenu is wired by RCTRootViewFactory — a code path consumers of this host bypass.
2. On older versions, the Fabric root view (RCTSurfaceHostingProxyRootView / RCTSurfaceHostingView) does not override menuForEvent: at all.

After creating the Fabric root view, this PR:

• 0.81+ path: runtime-checks [rootView respondsToSelector:@selector(setDevMenu:)], then resolves RCTDevMenu via -usingModule:block: and assigns it via performSelector:. This mirrors what RCTRootViewFactory.mm does upstream.
• Pre-0.81 fallback: installs an NSClickGestureRecognizer with buttonMask = 1 << 1 whose action fetches RCTDevMenu and calls [NSMenu popUpContextMenu:withEvent:forView:] with [devMenu menu].

Both paths are gated by:

#if defined(RCT_DEV_MENU) && RCT_DEV_MENU && __has_include(<React/RCTDevMenu.h>)
#define RNX_WIRE_DEV_MENU 1
#endif
...
#if defined(RNX_WIRE_DEV_MENU) && TARGET_OS_OSX

so iOS and release builds compile the wiring out entirely. The use of respondsToSelector: + performSelector: (rather than importing <React/RCTSurfaceHostingView.h> and casting) keeps the file compiling against react-native-macos versions that don't have the devMenu property in their public headers.

Relationship to microsoft/react-native-test-app#2757

This is the proper, upstream-aligned fix of a pair. microsoft/react-native-test-app#2757 is a self-contained workaround landed in the test app while this change makes its way through review and into a release; once this ships and the test app picks up the new @rnx-kit/react-native-host, that workaround can be removed.

The two surface different menus on secondary-click:

• This PR shows the standard RN dev menu (Reload, Open Debugger, …) — same UX as old-arch RCTRootView and as iOS.
• fix(macos): show React menu on right-click of Fabric root views react-native-test-app#2757 shows the test app's own React menu (component picker + Load From Dev Server + …).

Any consumer of host.viewWithModuleName: benefits from this PR — not just the test app — which is why it belongs here in the host rather than papered over in each app.

Test plan

• Build against react-native-macos 0.75 (no setDevMenu:) — fallback gesture-recognizer path is exercised; right-click the React content shows the dev menu.
• Build against react-native-macos 0.81+ (has setDevMenu:) — property-wiring path is exercised; right-click shows the dev menu.
• Build for iOS — RNX_WIRE_DEV_MENU block is gated by TARGET_OS_OSX, no behavior change.
• Build a release configuration — RCT_DEV_MENU is 0, the entire block is compiled out.

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