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

Services/index.md

@@ -16,6 +16,9 @@ External-facing logic: YouTube API, persistence. See
 | `AlertClipDecoder.cs` | **Per-play alert video decoder (TASK 47):** `IAlertClipDecoder` (Start/Stop/Dispose + `FrameAvailable`/`AudioReady`/`Completed`) + ffmpeg child emitting `rawvideo bgra` (`-vf scale=W:H`, read via `RawVideoFrameReader`) and `f32le -ar 48000 -ac 2` on a **carry-buffer** audio loop (float boundary straddles never drop samples), both real-time paced; bgra is opaque (layer owns alpha). One-shot lifetime — created per alert, disposed at drain; tests inject a fake |
 | `LayoutStore.cs` | SQLite persistence (`Microsoft.Data.Sqlite`) at `%APPDATA%\ytLlive\ytLlive.db`; assets stored as BLOBs keyed by SHA-256 content hash; save/open layout files; schema `user_version` 10 (`Source.ClipShape`/`IsMirrored` via `ALTER TABLE` for pre-v2 DBs; v3 = singleton `Webcam` + per-scene `WebcamSceneConfig`, migrated idempotently **without backfill** — the stale `Source.DeviceId` column remains but is no longer read/written; v4 = `WebcamSceneConfig.RectWidth`/`RectHeight`, the pre-Round rect for the round-to-rect restore; v5 = `Source.IsBackdrop` + `Source.CaptureKey`, the live-capture backdrop; v6 = `Scene.HasBackdrop` — Live-only policy, one-time backfill turns Starting/BRB/Chat/Ending off + drops their backdrop sources; v9→v10 (TASK 39) = `YppSnapshot` history table; all later columns stay additive guarded ALTERs — TASK 47 adds the five alert `Source` columns (`AlertVideoAssetId`/`AlertVideoPath`/`AlertUseDefaultVideo`/`AlertShowTicker`/`AlertVideoVolume`) without a version bump; `MainViewModel.NormalizeBackgrounds` re-normalizes on every load — TASK 25: one locked "Background" per canonical screen (static art everywhere, DisplayCapture on Live), duplicates dropped, index 0, orphaned `BackgroundUseDefault_*`/`BackgroundPath_*` Settings keys purged at save). Settings key/value table holds app state: `MicSourceName`, `ReusableStream` (TASK 9 — the cached reusable stream's id/address/name so the pump has its RTMP URL at startup), `AlertDefaultVideoAssetId` (TASK 47 — the stamped built-in alert clip's asset row; save's prune exempts it via UNION) |
 | `VideoFrame.cs` | Normalized CPU frame seam (`Width`/`Height`/tightly-packed BGRA `byte[]`) — the only pixel type the rest of the app knows about; future capture sources (screen, background-removed webcam) feed the same seam |
+| `BackgroundEffects/IPersonSegmenter.cs` | **TASK 51 seam:** `Segment(VideoFrame) → SegmentationMask?` (`SegmentationMask(Width, Height, float[] Values)`, 0 = background, 1 = person); null = no mask → caller passes through. Tests inject fakes |
+| `BackgroundEffects/OnnxPersonSegmenter.cs` | **TASK 51 default segmenter:** MediaPipe Selfie Segmenter (landscape, Apache-2.0) as an embedded ONNX resource, run by onnxruntime on the CPU (2 intra-op threads); frame stretched to 256×144 RGB/255 NHWC; lazy session; never throws (fail → log once, null forever) |
+| `BackgroundEffects/BackgroundEffectProcessor.cs` | **TASK 51 pure pixel stage:** `Apply(frame, BackgroundEffect)` — None = same instance, Remove = copy with alpha = bilinear mask × source alpha (`IsOpaque=false`), Blur = passthrough until slice 2; no mask/exception → passthrough + one log. Not wired into the camera path yet |
 | `CameraDeviceInfo.cs` | `(Id, DisplayName)` for a physical capture device |
 | `ICameraEnumerator.cs` | `GetCamerasAsync()` — seam so the picker/`CameraManager` never touch WinRT (tests inject fakes) |
 | `ICameraFrameSource.cs` | `StartAsync`/`StopAsync`/`FrameAvailable(VideoFrame)` + **`SourceFailed(string)`** — seam for a running capture source (tests inject fakes) |