Upgrade Avalonia projects to v12

[highbyte]

Summary

Upgrades Avalonia from 11.3.x to 12.0.1 across all six Avalonia projects (Core library + Desktop/Browser/iOS/Android apps).

Priority per the upgrade plan: Desktop and Browser must run; iOS and Android only need to compile.

Package changes

• All 8 Avalonia.* packages → 12.0.1 (including Avalonia.Skia, Avalonia.Android, Avalonia.iOS, Avalonia.Browser).
• SkiaSharp native-asset packages bumped to the 3.119.3-preview.1.1 line (Avalonia 12 Skia requires it).
• Dropped Avalonia.Diagnostics (no 12.x release yet — temporarily removes Ctrl+F12 DevTools in Debug desktop builds).
• Dropped Avalonia.ReactiveUI (deprecated for v12) and replaced it with the ReactiveUI core package (not deprecated, actively maintained).
• Removed the unused CommunityToolkit.Mvvm reference.

ReactiveUI migration

Avalonia.ReactiveUI's only real contribution in this codebase was registering AvaloniaScheduler as RxApp.MainThreadScheduler via .UseReactiveUI(). It's replaced with:

• A custom AvaloniaDispatcherScheduler shim (~30 lines, posts to Dispatcher.UIThread).
• Explicit ReactiveUI 23.x bootstrap via RxAppBuilder.CreateReactiveUIBuilder()...BuildApp() in App.OnFrameworkInitializationCompleted. (ReactiveUI 23 no longer auto-initializes — first use of ReactiveObject throws TypeInitializationException without this.)
• Find/replace across 8 ViewModels: RxApp.MainThreadScheduler → RxSchedulers.MainThreadScheduler (the RxApp class was split into RxSchedulers / RxState in ReactiveUI 23).
• Browser .csproj adds <TrimmerRootAssembly Include="ReactiveUI" /> — the WASM AOT trimmer otherwise strips RxSchedulers / RxAppBuilder and the app throws TypeLoadException at load time.

All existing ReactiveObject / RaiseAndSetIfChanged / ReactiveCommand / ObservableAsPropertyHelper usage is unchanged.

v12 breaking-change fixes

• App.axaml.cs: removed the DisableAvaloniaDataAnnotationValidation method and its call — the data-annotations binding plugin was removed in v12 (compiled bindings are now the default).
• C64MenuView.axaml.cs: migrated clipboard from GetTextAsync / SetTextAsync to the new IAsyncDataTransfer / DataTransferItem API.
• OverlayDialogHelper.cs: GetVisualRoot() → TopLevel.GetTopLevel(...) (the visual-tree helper extension is gone).
• MainWindow.axaml.cs: removed InitializeComponent(attachDevTools: false) — the parameter was removed in v12.
• Android MainActivity.cs: rewritten for the v12 model — non-generic AvaloniaMainActivity + a new AvaloniaAndroidApplication<App> subclass hosting CustomizeAppBuilder.
• iOS/Android .csproj target frameworks: net9.0-ios / net9.0-android → net10.0-ios / net10.0-android (Avalonia 12 mobile requires .NET 10).

Test plan

• Full solution build succeeds (0 errors, Debug & Release).
• Desktop app launches, menu opens, clipboard copy+paste works, a C64 system loads & runs.
• Browser app boots in a browser, main view renders, Lua script panel works.
• iOS/Android only verified to compile — no runtime test required this cycle (not actively used).

Known follow-ups

• Re-add Avalonia.Diagnostics (DevTools) once a 12.x release is published — marker comment left in Core.csproj.
• SkiaSharp.NativeAssets.* are pinned to a prerelease; bump to stable when one compatible with Avalonia 12 ships.
• The TrimmerRootAssembly Include="ReactiveUI" in the Browser project preserves ~150KB of ReactiveUI IL from trimming. If WASM payload size becomes a concern, this is the first thing to revisit.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud