Add CI workflow to build mobile /samples heads: Android, iOS, Uno (Phase 3 of #509)

[adrianhall]

Summary

Follow-up to #509 (Phase 3 of the samples CI rollout). This covers the mobile build heads (Android/iOS/MAUI/Uno) that require workload installation and, for iOS, a macOS runner.

Scope

Sample	Solution / Project(s)	TFMs	Runner / workload needs
Avalonia TodoApp	samples/todoapp/TodoApp.Avalonia/TodoApp.Avalonia.Android/TodoApp.Avalonia.Android.csproj	net10.0-android	android workload
Avalonia TodoApp	samples/todoapp/TodoApp.Avalonia/TodoApp.Avalonia.iOS/TodoApp.Avalonia.iOS.csproj	net10.0-ios	ios workload + macOS/Xcode runner
Uno TodoApp	samples/todoapp/TodoApp.Uno/TodoApp.Uno.sln	net10.0-android;net10.0-ios;net10.0-maccatalyst;net10.0-windows10.0.26100;net10.0-browserwasm;net10.0-desktop	Uno.Sdk (pinned via global.json, currently 5.4.5) + android/ios/maui/wasm-tools workloads across runners
MAUI TodoApp	samples/todoapp/TodoApp.MAUI/TodoApp.MAUI.csproj	net10.0-android;net10.0-ios	maui/android/ios workloads

Suggested approach

Start with Avalonia mobile heads first (simplest — 2 heads, no Uno.Sdk complications), then MAUI Android/iOS, before attempting Uno.

Note: the Uno desktop head currently has a pre-existing NU1605 (Uno.WinUI version mismatch) per #506 that will need resolving before that head builds cleanly in CI. Consider continue-on-error: true (or excluding entirely) for any head known to have a pre-existing, unrelated build break, so this workflow doesn't start life red for reasons outside a given PR's control.

Should be build-only (dotnet restore + dotnet build), consistent with Phases 1 and 2.

Related

• Add CI workflow to build all /samples projects on PR #509 — Phase 1 (ubuntu-only, no-workload samples).
• Add CI workflow to build Windows-only /samples heads (Phase 2 of #509) #510 — Phase 2 (Windows-only heads).
• Investigate resolving NU1903 for Tmds.DBus in Uno desktop sample #506 — Uno Tmds.DBus NU1903 investigation; blocked by the pre-existing Uno.WinUI NU1605 that this phase would also need to contend with.