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

Models/BackgroundEffect.cs

@@ -0,0 +1,11 @@
+namespace ytLive.Models;
+
+/// <summary>Per-webcam background effect (TASK 51). <see cref="Remove"/> makes everything
+/// outside the person transparent so the layers below show through; <see cref="Blur"/>
+/// blurs the background behind the person.</summary>
+public enum BackgroundEffect
+{
+    None,
+    Remove,
+    Blur,
+}