Fix Xcode 27 actor isolation build error

[ShityLife]

Summary

• replace platform application lifecycle notification properties with their equivalent notification names
• avoid accessing actor-isolated UIKit/AppKit/WatchKit members from a nonisolated global initializer
• preserve the existing foreground synchronization behavior on every supported Apple platform

Problem

Xcode 27 Beta fails to compile iCloudKVKey.swift in Swift 6 mode with:

Main actor-isolated default value in a nonisolated context

The failure occurs while initializing willEnterForegroundNotificationName.

Validation

• swift build succeeds with Xcode 27 Beta and swift-sharing 2.9.1
• verified in an iOS 27 simulator consumer app build and launch

[KthKuang]

Thanks for the report and the patch. I can't reproduce this with the current stable toolchain: with Xcode 26.6 in Swift 6 language mode, the package builds cleanly for macOS, iOS, tvOS, and watchOS, and the iOS 26.5 SDK explicitly annotates UIApplicationWillEnterForegroundNotification as NS_SWIFT_NONISOLATED, so accessing it from a nonisolated context is intentional and allowed today.

Since beta SDK annotations are still in flux, I'd rather not work around a diagnostic that may not survive to release. Let's revisit this when Xcode 27 reaches RC: if the error is still present at that point, I'm happy to fix it — though the change will need to be redone on top of current main, since #5 rewrote iCloudKVKey.swift and this branch no longer merges cleanly.

In the meantime, sharing the exact Xcode 27 beta version and the full compiler output would help confirm the diagnosis.