TASK 51 slice 1: background-effects engine — bundled MediaPipe Selfie Segmenter (ONNX) + Remove (Unit: bg-effects engine)

- Model: Google's official MediaPipe Selfie Segmenter (landscape, Apache-2.0 per model card),
  converted to ONNX by tools/selfie-seg/convert_tflite_to_onnx.py (custom op
  Convolution2DTransposeBias -> ConvTranspose) and verified against MediaPipe's own runtime
  (verify_against_mediapipe.py: max |diff| 5e-5, 100% binary agreement). Embedded resource, 430 KB.
- Services/BackgroundEffects: IPersonSegmenter seam, OnnxPersonSegmenter (onnxruntime 1.31.0 CPU,
  never throws), BackgroundEffectProcessor (None passthrough, Remove = alpha mask). Not wired yet.
- Tests: Remove integration test (fake segmenter) + bundled-model contract test vs MediaPipe
  reference mask. Full suite 407/408 — the one failure is the documented interactive-desktop flake
  RealMouseDrag_OnTheLayerList_PersistsTheReorder (passed in the run before; code path untouched).
- Docs: ai.md reversal + section + licensing guard, THIRD-PARTY-NOTICES 11-12, TASK 51, indexes,
  MyMistakes recipe, HANDOFF.

References: obs-backgroundremoval (approach only — ONNX Runtime on CPU, 2 threads; GPL-3.0, no code
used) https://github.com/royshil/obs-backgroundremoval ; MediaPipe image segmenter
https://ai.google.dev/edge/mediapipe/solutions/vision/image_segmenter

ytLive.csproj

@@ -75,6 +75,14 @@
     <None Include="THIRD-PARTY-NOTICES.txt" CopyToOutputDirectory="PreserveNewest" />
   </ItemGroup>
 
+  <!-- Webcam background effects (TASK 51): Google's MediaPipe Selfie Segmenter (landscape,
+       Apache-2.0), converted to ONNX by tools/selfie-seg/ and embedded in the assembly —
+       ~430 KB, so it lives in git, ships inside the exe, and is never downloaded at runtime
+       (Store policy 10.2.2, same posture as the bundled ffmpeg). -->
+  <ItemGroup>
+    <EmbeddedResource Include="Assets\Models\selfie_segmenter_landscape.onnx" LogicalName="ytLive.Assets.Models.selfie_segmenter_landscape.onnx" />
+  </ItemGroup>
+
   <!-- Bundled FFmpeg (TASK 48 item 1, Store policy 10.2.2 dynamic code inclusion).
        The app used to DOWNLOAD an unsigned ffmpeg from GitHub at first run and execute
        it; that 404'd on the creator's first real recording (2026-09-01) and is the one
@@ -102,6 +110,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.Data.Sqlite" Version="8.0.10" />
+    <PackageReference Include="Microsoft.ML.OnnxRuntime" Version="1.31.0" />
     <PackageReference Include="NAudio.Wasapi" Version="2.2.1" />
     <PackageReference Include="NAudio.WinMM" Version="2.2.1" />
     <PackageReference Include="Notification.Wpf" Version="11.0.0" />