Add .NET 11 Passkeys sample

[davidortinau]

Summary

• add a .NET MAUI 11 Passkeys client for Android, iOS, Mac Catalyst, and Windows
• add a minimal ASP.NET Core Identity relying-party server that validates WebAuthn ceremonies
• add deterministic dev-tunnel, Apple AASA, and Android Digital Asset Links setup
• document platform prerequisites, the security boundary, and troubleshooting

Dependency

#775 merged the .NET 11 Preview 7 SDK and MAUI sample baseline. #778 pinned PR CI to the exact 11.0.100-preview.7.26381.103 SDK. #779 updated shared CI to Xcode 26.6, as required by the Preview 7 iOS and Mac Catalyst workloads. This branch is rebased onto those changes and the latest upstream main.

Validation

• built the server and MAUI client for Android, iOS Simulator, and Mac Catalyst
• exercised live health, AASA, Digital Asset Links, account, and passkey ceremony endpoints
• verified the iOS simulator UI with MAUI DevFlow
• verified that setup generates the local Associated Domains entitlement from the empty committed iOS base plist
• reviewed origin validation, generated local configuration, app identity persistence, and the final entitlement fix

Final CI status

All required Preview 7 packages are public on NuGet.org. After #778 and #779 fixed the shared SDK and Xcode setup, CI exposed an unnecessary committed iOS keychain-access-groups entitlement. That made the hosted iOS Simulator build require a real Apple signing certificate. The base iOS plist is now empty, because the Passkeys API does not need that keychain group; Configure-Passkeys.ps1 still generates the ignored local Associated Domains entitlement for configured Apple builds.

The entitlement-fix run 31527010567 passed macOS, Windows, and CLA. The final ancestry-only rebase onto current main was proven byte-for-byte unchanged, and fresh run 31527938252 passed macOS and Windows. The PR is ready for review.

[davidortinau]

@jfversluis Final release-day sample is green on macOS and Windows at 0c653fd1. The last CI fix removes an unnecessary committed iOS keychain entitlement while preserving generated local Associated Domains; the setup script and unsigned simulator build were revalidated. Passkeys docs are already published, so please review and merge to resolve the sample link.

[davidortinau]

@conceptdev Release-day final dependency: #776 is green on macOS/Windows, current with main, and the Passkeys docs are already published. If available, please review and merge.