[Bugfix] Keep the editor's menu bar when SwiftUI installs its own

[miogds]

What happened

Since #153 the editor is a SwiftUI App, so that it can declare the space the Apple Vision Pro preview shows, while its menus are still built by the AppKit delegate at launch. SwiftUI installs a menu bar of its own from its default commands. On macOS 26 it does so after the delegate has set the editor's, and the bar then reads Untold Engine Studio · Edit · View · Window · Help: no File menu, none of the editor's View items, the app menu named after the Info.plist. On macOS 27 the editor's menu survives whatever happens after launch (checked 8 s in, after activation, after the Settings scene, after asking for the preview), which is why the change was not seen before the merge.

The fix

EditorMainMenuKeeper (Editor/Window/) does not fight over which NSMenu is the main menu. It moves the editor's menus into whatever menu bar is installed, takes that bar's own items out, drops the delegate that fills it in, and keeps the Window menu the editor's, so SwiftUI keeps the object it installed and the user sees the editor's menus. It looks again whenever the main menu changes and after every event the application handles (NSApplication.didUpdate, didBecomeActive, NSWindow.didBecomeKey), so it depends neither on the setter being observable nor on winning a replace war. AppDelegate keeps one instead of assigning NSApp.mainMenu directly.

The first commit put the editor's own NSMenu back through KVO; that did not hold on macOS 26, so the second commit takes the approach above.

Verification

• EditorMainMenuKeeperTests: nine tests on the real NSApplication: a bar installed before the editor's menus, one installed later, one refilled in place without a setter running, one that fills itself in through its delegate, the Window menu, and one showing that without the keeper another bar stays.
• Full editor suite: 1243 tests, 0 failures. SwiftFormat clean on the two new files.
• The symptom only shows on macOS 26, which I cannot run here: @untoldengine, could you confirm the File menu and the View items are back on your Mac? The log should show one line, The menu bar was replaced; the editor's menus are put back.