Replace NativeCloudKitService with CloudKit framework in MistDemoApp

[leogdion]

Context

MistDemoApp (Examples/MistDemo/Sources/MistDemoApp) currently routes its CloudKit operations through NativeCloudKitService, which is built on top of MistKit's REST-based CloudKitService. On Apple platforms, applications should use the CloudKit framework (import CloudKit, CKContainer, CKDatabase) directly — MistKit's value lives in server/Linux/WASI/Windows contexts where the CloudKit framework is unavailable.

Why

• Honest pitch: the right tool for the platform. The native example shouldn't double as a MistKit demo when CloudKit framework is the natural choice on iOS/macOS.
• Removes a confusing signal — users browsing the example shouldn't conclude that MistKit is the recommended way to talk to CloudKit from a SwiftUI app.
• Simplifies auth in the app: signed-in iCloud user, no token plumbing. The public/private database picker becomes container.publicCloudDatabase vs container.privateCloudDatabase, which absorbs the app side of Add public database interface to MistDemoApp and web #275.

What

• Replace (or rewrite) NativeCloudKitService with a CloudKit-framework-based service using CKContainer (default or the container ID configured in entitlements), CKDatabase, CKRecord, CKQuery, CKQueryOperation, etc.
• Wire the SwiftUI views to the new service.
• Drop the auth-token UI from the app (no longer applicable on Apple platforms).
• Implement the public/private database selector in SwiftUI (absorbs the app half of Add public database interface to MistDemoApp and web #275).
• Confirm the app's Package.swift no longer depends on MistKit.

Out of scope

• Web demo and CLI continue to use MistKit; this issue only affects MistDemoApp.
• MistKit-on-Apple coverage continues via the package's own test target.

Related

• Supersedes the app side of Add public database interface to MistDemoApp and web #275; the web side of Add public database interface to MistDemoApp and web #275 remains.

[leogdion]

Resolved by #339 (commit 6be4939).