TASK 21 Increment B: FFmpeg rawvideo video decoder -> VideoFrame

Adds the codec-agnostic decoder half of the media source. Spawns ffmpeg
with -f rawvideo -pix_fmt bgra (reusing the already-shipped ffmpeg via
IFfmpegLocator) and drains the raw BGRA stdout pipe into VideoFrames.

- Services/RawVideoFrameReader.cs: pure rawvideo BGRA stream -> frames
  (partial reads kept across Feed; no ffmpeg needed to test)
- Services/IDecodeProcess.cs + FfmpegDecodeProcess.cs: binary-stdout
  subprocess seam, mirror of the encoder's IEncoderProcess
- Services/MediaVideoSource.cs: owns the decode, raises FrameReady/Completed
- MediaVideoSourceTests: 3 pure reader + 1 integration (fake decode
  process through the real source loop, frames in order)

Reference (external scan): ffmpeg rawvideo pipe decode is the canonical
codec-agnostic frame feeds pattern (ffmpeg docs -f rawvideo; how OBS/media
pipelines push frames to a compositor). Verified: 4/4 tests, build 0 warnings.
Native-FPS pacing + resolver/compositor wiring are the next slice.

ai.md

@@ -155,7 +155,7 @@ C# / WPF (.NET 8) following MVVM:
 |------|------|
 | `Models/` | Plain data types — Scene, Source (incl. `ClipShape`, `IsMirrored`, `VideoImageSource`), QualityOption, StreamConfig, StreamHealth, YouTubeChannel, ChatMessage, **Socials (`SocialService` enum + `SocialEntry`/`SocialsConfig` + `SocialServiceIcons`) — the social bar** |
 | `ViewModels/` | MainViewModel — `public partial class`, one file per functional area (Scenes, Background, Webcam, Audio, Trax, Socials, Streaming, Chat, Overlays, Account, License, Recording — split complete, see `ViewModels/index.md`); **Chat.cs is a thin delegating facade over `Services/ChatOverlayLayer.cs` (Commit G, first true decomposition)**; GoLiveViewModel, ReuseImageViewModel, CameraPickerViewModel, **SocialsDialogViewModel** |
-| `Services/` | YouTube OAuth2, stream/broadcast management, live chat polling, LayoutStore (SQLite), **SocialValidator (`ISocialValidator` seam + `HttpSocialValidator` default)**, **webcam: `VideoFrame` seam + `CameraDeviceInfo`/`ICameraEnumerator`/`ICameraFrameSource` interfaces + `MediaCaptureCameraEnumerator`/`MediaCaptureFrameSource` (WinRT) + `CameraManager`**, **screen capture: `IFullScreenDetector`/`Win32FullScreenDetector` + `IScreenCaptureSource`/`ScreenCaptureFrameSource` (WinRT GraphicsCapture) + `ScreenCaptureManager` + `ScreenCaptureSourceFactory` + `Direct3D11Helper`/`CaptureInterop` (COM bridges)**, **compositor: `SceneCompositor` + `CompositorOptions` + pure `StretchMath` + `StaticPixelCache` (see "Scene compositor")**, **audio: `IAudioSource` seam + `WasapiLoopbackAudioSource`/`WasapiMicAudioSource` (NAudio WASAPI) + `AudioMixer` + pure `AudioLevelMeter`/`WaveToFloat`/`VoiceFilterChain`/`LowShelfFilter`/`HighShelfFilter`/`NoiseGate`/`Compressor`/`AutoDucker`/`AudioRingBuffer`/`TinyResampler`/`AudioSyncDelay` + `MusicPlayer` + `IAudioPipeWriter`/`NamedPipeAudioWriter` (see "Live audio capture")**, **encoder: `IFfmpegEncoder`/`FfmpegEncoder` + `IEncoderProcess`/`FfmpegEncoderProcess` + `IFfmpegLocator`/`FfmpegLocator` + pure `FfmpegArgs`/`FfmpegProgressParser`/`FfmpegEncoderPicker` + the `FramePump` frame producer (see "Live encoder" + "Live frame pipeline")**, **notifications: `INotificationService` seam (`AppNotificationSeverity` Info/Success/Warning/Error) + `NotificationService` (Notification.Wpf toasts, see "Toast notifications")** |
+| `Services/` | YouTube OAuth2, stream/broadcast management, live chat polling, LayoutStore (SQLite), **SocialValidator (`ISocialValidator` seam + `HttpSocialValidator` default)**, **webcam: `VideoFrame` seam + `CameraDeviceInfo`/`ICameraEnumerator`/`ICameraFrameSource` interfaces + `MediaCaptureCameraEnumerator`/`MediaCaptureFrameSource` (WinRT) + `CameraManager`**, **screen capture: `IFullScreenDetector`/`Win32FullScreenDetector` + `IScreenCaptureSource`/`ScreenCaptureFrameSource` (WinRT GraphicsCapture) + `ScreenCaptureManager` + `ScreenCaptureSourceFactory` + `Direct3D11Helper`/`CaptureInterop` (COM bridges)**, **media source: `MediaVideoSource` (spawns ffmpeg `rawvideo` BGRA decode) + pure `RawVideoFrameReader` + `IDecodeProcess`/`FfmpegDecodeProcess` process seam (binary-stdout mirror of `IEncoderProcess`; see "Media source")**, **compositor: `SceneCompositor` + `CompositorOptions` + pure `StretchMath` + `StaticPixelCache` (see "Scene compositor")**, **audio: `IAudioSource` seam + `WasapiLoopbackAudioSource`/`WasapiMicAudioSource` (NAudio WASAPI) + `AudioMixer` + pure `AudioLevelMeter`/`WaveToFloat`/`VoiceFilterChain`/`LowShelfFilter`/`HighShelfFilter`/`NoiseGate`/`Compressor`/`AutoDucker`/`AudioRingBuffer`/`TinyResampler`/`AudioSyncDelay` + `MusicPlayer` + `IAudioPipeWriter`/`NamedPipeAudioWriter` (see "Live audio capture")**, **encoder: `IFfmpegEncoder`/`FfmpegEncoder` + `IEncoderProcess`/`FfmpegEncoderProcess` + `IFfmpegLocator`/`FfmpegLocator` + pure `FfmpegArgs`/`FfmpegProgressParser`/`FfmpegEncoderPicker` + the `FramePump` frame producer (see "Live encoder" + "Live frame pipeline")**, **notifications: `INotificationService` seam (`AppNotificationSeverity` Info/Success/Warning/Error) + `NotificationService` (Notification.Wpf toasts, see "Toast notifications")** |
 | `Helpers/` | ViewModelBase (INotifyPropertyChanged), RelayCommand, ImageCache, AppLog (file logger), FocusPreservingListBox, OAuthCredentials, **TokenStore (DPAPI session persistence)**, visibility converters |
 | `Themes/` | `Controls.xaml` — the single dark-theme source, merged once in `App.xaml` (see `Themes/index.md`) |
 | `MainWindow.xaml` | Dark theme; layout: top bar (controls), center (preview + live controls below), left (scenes/sources), right (chat), bottom (gear + stream stats + resolution) |
@@ -669,6 +669,14 @@ seam:** `Func<Scene?>`, `Func<SceneElement, VideoFrame?>` resolver, `Func<Compos
   background → `StaticPixelCache.Get(AssetId)`. `BuildCompositorOptions` rounds the VM's `OutputRect*`
   doubles to ints — the vertical 607.5 half-pixel crop rounds to a perfectly-centered **608** (`Math.Round`,
   ToEven); `BuildEncoderOptions` fills W×H/FPS/bitrate from the tier once the URL provider yields one.
+- **Media source decoder (TASK 21, Increment B — 2026-08-31):** `MediaVideoSource` (`Services/MediaVideoSource.cs`)
+  decodes a local file to `VideoFrame`s by spawning ffmpeg with
+  `-f rawvideo -pix_fmt bgra -vf scale=WxH -an` and draining the raw BGRA stdout pipe through the pure
+  `RawVideoFrameReader` (`Services/RawVideoFrameReader.cs`), raising `FrameReady` per frame. The subprocess sits
+  behind `IDecodeProcess`/`FfmpegDecodeProcess` (binary-stdout mirror of the encoder's `IEncoderProcess`), and
+  the path comes from `IFfmpegLocator` — so it's codec-agnostic, uses the ffmpeg already shipped, and is
+  testable with a fake process. Frames emit as fast as the pipe produces them; native-FPS pacing (ffprobe probe,
+  wired by the caller) and the resolver/compositor wiring (`ResolveOutputFrame` case) are the next slice.
 - **Social bar on the output (bar bug-fix branch):** the `FramePump` takes an optional
   `socialBar: Func<(VideoFrame? Frame, SocialBarPosition Position)>?` seam, re-read **every frame** (so a
   mid-stream position flip applies immediately). The strip is pre-rasterized by `Compositor/SocialBarRenderer.cs`