Add composable GPU Privacy Shield sample

[mfkl]

Summary

• add the Native AOT Privacy Shield video-filter sample with persistent GPU redaction
• support solid, mosaic, and blur effects without modifying decoder reference surfaces
• compose YOLOX COCO-80, Open Model Zoo face, and Open Model Zoo license-plate detector profiles
• add SSD decoding, privacy vocabulary, per-detector tracking, and merged redaction masks
• add checksum-verified face/plate model downloads plus setup, limitations, and troubleshooting docs

Validation

• dotnet build vlclr.sln -c Release --no-restore — 0 warnings, 0 errors
• dotnet test vlclr.sln -c Release --no-build --no-restore — all test projects passed
• dotnet test tests/VLCLR.ObjectDetection.Tests -c Release --no-restore — 58 passed
• dotnet publish samples/PrivacyShield -c Release -r win-x64 --no-restore — Native AOT passed
• dumpbin /exports — vlc_entry and metadata exports present
• VLC 4 D3D11/OpenVINO integration — face and license-plate inference/redaction visually verified; solid, mosaic, and blur captures verified