feat: complete LearnMaui stages 1 through 28

[deku2026]

Summary

• map all 28 existing LearnMaui projects to the matching MauiStudy documents, from beginner through expert topics
• add a shared searchable learning surface plus runnable labs for each stage
• implement real MAUI examples for DI/lifecycle/async, XAML/MVVM/Shell, handlers, device APIs, networking/offline storage, accessibility, diagnostics, trimming, packaging, HybridWebView, migration, and runtime internals
• add the complete document-to-project mapping in src/LearnMaui/README.md

Validation

• built every Maui_Stage01 through Maui_Stage28 project individually for net10.0-android Debug with Visual Studio MSBuild (/restore /t:Build)
• confirmed XAML SourceGen builds for the stages that enable it
• rebuilt the repository C++ project successfully through a short path after diagnosing the Windows temporary-object path limit
• git diff --check

Android/MAUI remains outside the repository CI solution, matching the existing CI policy.