feat: fill LearnAvalonia 32 projects (Avalonia 12 desktop mainline)

[deku2026]

Fill 32 of 35 LearnAvalonia projects as runnable Avalonia 12.1 desktop demo apps, one demo page per knowledge-point section in the matching AvaloniaStudy doc.

Scope

• 32 projects filled (492 demo pages total), 1:1 mapped to ArchitectureDesign/AvaloniaStudy/ stage/Capstone docs (入门→专家级, 全覆盖).
• 3 skipped (Stage18 Mobile / Stage19 WebAssembly / Stage20 EmbeddedLinux): desktop-only policy; kept skeleton + README noting workload requirements.
• Capstone3 (was 全平台) adapted to desktop three-platform (Win/macOS/Linux).
• Stage24 adds Avalonia_Stage24_TestingSystem.Tests (Headless.XUnit + xUnit v3): 19 tests pass.

Architecture (方案 A: nav shell + 实验页)

Each project is an independent runnable Avalonia desktop app with a unified nav shell: left ListBox of knowledge points → right ContentControl showing the demo UserControl for the selected point. Per-project MainViewModel/TopicItem on CommunityToolkit.Mvvm 8.4.2. No cross-project shared library (matches LearnBlazor convention). CPM centralizes all package versions.

Per demo .axaml.cs carries a header comment block: Doc: (source section) / Title: / Stage: / Accept: (acceptance criterion) / Notes:.

Per-project demo counts

Project	demos	Project	demos
Stage01 AppBuilder	15	Stage21 AppArchitecture	20
Stage02 Dispatcher	15	Stage22 DataNetworkAuth	19
Stage03 AvaloniaXaml	16	Capstone2 ComplexDesktop	15
Stage04 PropertySystem	18	Stage23 AccessibilityI18n	12
Stage05 LogicalVisualTree	18	Stage24 TestingSystem	14
Stage06 LayoutSystem	17	Stage25 Performance	11
Stage07 ControlSystem	16	Stage26 TrimmingAOT	9
Stage08 StylesThemes	14	Stage27 PackagingCI	10
Stage09 DataBindingMVVM	16	Stage28 WpfMigration	8
Capstone1 DesktopFoundation	10	Capstone3 (desktop 3-platform)	10
Stage10 WindowsDialogs	19	Stage29 SourceReading	18
Stage11 PagesNavigation	18	Stage30 PlatformBackend	11
Stage12 InputFocusGestures	18	Stage31 RenderingExpert	17
Stage13 GraphicsAnimation	21	Capstone4 CustomControlAOT	15
Stage14 CustomControls	20
Stage15 Virtualization	17	Total	492
Stage16 PlatformAbstraction	18
Stage17 WebViewHybrid	17

Verification

• \dotnet build LearnCSharp.CI.slnx -c Release\ → 0 warnings / 0 errors
• \dotnet test src/LearnAvalonia/Avalonia_Stage24_TestingSystem.Tests\ → 19/19 pass
• \pre-commit run --all-files\ → all hooks pass (trailing-whitespace, end-of-file-fixer, mixed-line-ending, check-added-large-files, codespell, dotnet-format, clang-format)

Avalonia 12.1 compliance

Per roadmap §1.4 + 附录 B: no \Avalonia.ReactiveUI\ 11.x, no \UseDirect2D1, no old FileDialog (use \IStorageProvider), compiled bindings default on (\x:DataType), clipboard async \IAsyncDataTransfer, \TrayIcon+\\NativeMenu\, \SystemDecorations\\→\\WindowDecorations\, \WindowState\\ DirectProperty. DataGrid marked deprecated; TableView core; TreeDataGrid noted as Pro/commercial.

Design & plan

• Spec: \docs/superpowers/specs/2026-07-27-learnavalonia-fill-design.md\
• Plan: \docs/superpowers/plans/2026-07-27-learnavalonia-fill.md\

All work in worktree .worktree/learnavalonia-fill\ on branch \learnavalonia-fill\ per user instruction.