Consolidate stable and experimental Windows AI samples

[shisan (qiutongMS)]

Description

Consolidates the stable and experimental Windows AI Foundry scenarios onto main without maintaining a separate source tree. Stable functionality is integrated into the core sample, while functionality that is still experimental remains isolated and version-gated.

This change:

• Introduces WindowsAppSDKVersion as the canonical Windows App SDK version updated by UpdateVersions.ps1.
• Derives the package channel from that version and uses it to include experimental extensions.
• Extracts code shared by stable and experimental scenarios into WindowsAISample.Shared.
• Integrates Video Scaler into the core sample because it is now a stable API, following Add back Video Super Res sample, update to WASDK 2.4.0 #669.
• Keeps Image Foreground Extractor in a separate extension project that is built only for an experimental Windows App SDK package.
• Removes stale extension artifacts when switching from an experimental build back to a stable build.
• Documents the stable and experimental behavior in the Windows AI Foundry sample README.

Validation

• Windows AI Foundry x64 Release with stable 2.4.0: passed with zero warnings and errors.
• Windows AI Foundry x64 Release with 2.4.1-experimental: passed with zero warnings and errors.
• Experimental-to-stable channel switch: passed; stale extension output was removed.
• The Aggregator pipeline passed here: Build 158111240

[shisan (qiutongMS)]

The stable VideoScaler baseline from #669 is being synced to main in #673. After #673 merges, this PR will be rebased so its diff focuses on the shared-project and experimental ImageForegroundExtractor extension architecture.