feat: Phase 1a : MAUI alpha on .NET 10 (44 components + tests)

[Shewart]

Summary

Takes shellui-native from the 6-month-stale skeleton on main (3 components, .NET 8, docs stubs)
to a working MAUI alpha on .NET 10 with 44 registered components, a compositional overlay
pattern, an xUnit test suite (147 tests), and an Avalonia-ready platform model. See
docs/DEVELOPMENT_PLAN.md for the phased branch strategy and full
Phase 1a checklist.

Scope: 44 files changed (39 modified, 5 new). Cold multi-TFM build is 0 warnings / 0 errors
across net10.0-{android,ios,maccatalyst,windows}. dotnet test is 147/147.

What's new

Platform / infra

• All projects on .NET 10 — Core, Templates, CLI, tests, and the MAUI demo
• global.json pins SDK 10.0.100 with rollForward: latestFeature
• Solution migrated from .sln → .slnx (XML-format, post-.NET-9)
• CI (ci.yml, pr-check.yml, release.yml) upgraded to dotnet-version: 10.0.x; the
  build-examples job builds MAUI.Demo end-to-end on the Windows head with the MAUI workload
• CI now runs dotnet test and uploads TRX + coverage artifacts
• MAUI demo .csproj: WindowsPackageType=None, explicit RuntimeIdentifier=win-x64, explicit
  Microsoft.Maui.Controls + Microsoft.Extensions.Logging.Debug PackageReferences

Platform model — Avalonia-ready

• NativePlatform.Avalonia added to the enum
• Avalonia project detection via PackageReference prefix or App.axaml presence
• Avalonia option in the init prompt and platform-name display
• CLI description reflects "MAUI or Avalonia"
• ComponentInstaller warns when the target platform isn't MAUI (templates are MAUI-only until
  Phase 1b — Template System v2 — lands)

Components (44 templates, see docs/COMPONENTS_ROADMAP.md)

• P0 complete: shell, button, input, label, checkbox, switch, card (+ header/content/footer), separator, badge, progress, alert
• P1 complete: dialog (+ trigger/content/header/footer/title/description/close), drawer (+ trigger/content), sheet (+ trigger/content), dropdown (+ trigger/content/item), popover (+ trigger/content)
• P2 partial: textarea, slider, select, radio-group (+ item), date-picker, time-picker
• All overlays use the compositional Parent + Trigger + Content pattern via the auto-installed element-extensions utility (Element.FindParentOfType<T>())

Template hygiene (regression fixes)

• All 18 RoundRectangle-using templates now include using Microsoft.Maui.Controls.Shapes; in their Content string (was silently producing non-compiling code on install)
• All 14 compositional-container Children properties (Card, CardFooter, Dialog{Content,Header,Footer}, DrawerContent, SheetContent, PopoverContent, DropdownContent, RadioGroup) marked new to silence CS0108 against TemplatedView.Children
• Switch.IsEnabledProperty and Checkbox.IsEnabledProperty marked static new (intentional shadows of VisualElement.IsEnabledProperty)
• Switch uses _ = TranslateToAsync(...) instead of the obsolete TranslateTo (CS0618)

Tests — new tests/ShellUI.Native.Tests

• xUnit + Microsoft.NET.Test.Sdk + coverlet, mirroring shellui-blazor's setup
• 147 tests covering:
  • Every registered component has non-empty Content (all 44 templates)
  • Every template using RoundRectangle imports Microsoft.Maui.Controls.Shapes (regression lock)
  • Every template has the YourProjectNamespace placeholder
  • Every metadata Name matches its registry key
  • Every declared dependency is itself registered (catches typos)
  • ProjectDetector.DetectPlatform maps .csproj XML → NativePlatform for MAUI / Avalonia (both PackageReference and App.axaml paths) / WinUI / WPF / Unknown, plus the MAUI-over-Avalonia precedence edge case
  • NativePlatform.Unknown is default
• DetectPlatform promoted private → internal + InternalsVisibleTo so tests hit the pure XML→enum core without cwd manipulation

Docs

• New DEVELOPMENT_PLAN.md — branch strategy, phase deliverables + exit criteria, cross-cutting concerns
• PLAN.md — WPF → Avalonia phase-2 revision with rationale, Template System v2 section, .NET 10 specs
• ARCHITECTURE.md — Avalonia detection table + limitation note, project structure with tests/, theming table, planned Avalonia TemplatedControl pattern example
• COMPONENTS.md — reconciled with the registry: removed stale "Coming Soon" entries, added docs for the 6 P2 form components, updated category summary
• QUICKSTART.md — .NET 10.0 SDK prereq, platform prereq updated for MAUI-today / Avalonia-Phase-2
• README.md — Supported Platforms table now MAUI (active) / Avalonia (Phase 2) / WinUI (conditional Phase 3); WPF is detection-only

Verification

• dotnet build ShellUI.Native.slnx -c Release → 0 warnings, 0 errors (cold multi-TFM, ~13min)
• dotnet test tests/ShellUI.Native.Tests -c Release → 147/147
• dotnet build examples/MAUI.Demo/MAUI.Demo.csproj --framework net10.0-windows10.0.19041.0 → green

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