TASK 4 ship step 5: live frame pipeline — FramePump paces the active scene's composite into the encoder, ScreenCaptureManager.GetLatestFrame, MainViewModel resolver/option-builder/pump wiring, FramePumpTests (7) + GetLatestFrame test — 147 tests passing, 0 warnings

ai.md

@@ -91,7 +91,7 @@ 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 — exposes collections + commands for the UI; 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)**, **audio: `IAudioSource` seam + `WasapiLoopbackAudioSource`/`WasapiMicAudioSource` (NAudio WASAPI) + `AudioMixer` + pure `AudioLevelMeter`/`WaveToFloat` (see "Live audio capture")** |
+| `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` (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")** |
 | `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), left (scenes/sources), right (chat), bottom (health) |
@@ -352,7 +352,8 @@ integration test fakes the whole subprocess (probe + encoder) with a Channel-bac
 **Encoder choice is probed from the binary's `-encoders` listing** (`FfmpegEncoderPicker`, pure):
 NVENC → QSV → AMF → OpenH264 fallback, **never libx264** (GPL; see Licensing). `EncoderOptions.VideoEncoder`
 forces one and skips the probe. `EncoderOptions` also carries W×H/FPS/bitrate from the quality tier and
-`GopSize` = FPS×4. Not yet constructed by the app — the frame-pipeline wiring (ship step 5) owns it.
+`GopSize` = FPS×4. Constructed by `MainViewModel` since ship step 5 (`new FfmpegEncoder(new FfmpegLocator())`),
+driven by the `FramePump` below.
 
 ### Live audio capture (TASK 4 ship step 4 — shipped 2026-08-12, plan in TASKS.md)
 
@@ -371,16 +372,44 @@ timers).
 - **`AudioMixer`** owns both sources; `Start`/`Stop` follow go-live (`MainViewModel.BeginGoLive` success
   → `_audioMixer.Start()`, `StopStream` → `Stop()`). Mic samples feed a pure **`AudioLevelMeter`** (RMS
   with 0.2 exponential smoothing) → `MicLevelChanged` → marshalled to the UI thread → `AudioLevel`.
-  Desktop samples are currently **dropped** — the future encoder's AAC mix (ship step 5) consumes them,
-  replacing the `-f lavfi -i anullsrc` placeholder. Failures log via `AppLog`; a mic failure zeroes the
+  Desktop samples are currently **dropped** — a later step's AAC mix consumes them, replacing the
+  `-f lavfi -i anullsrc` placeholder (the encoder construction itself shipped in ship step 5). Failures log via `AppLog`; a mic failure zeroes the
   meter, a loopback failure never kills the mic.
 - **`WaveToFloat`** (pure, shared): WASAPI mix formats → interleaved float — IEEE float 32-bit direct,
   PCM 16-bit normalized to -1..1, `WaveFormatExtensible` with the IEEE-float subformat GUID
   (`NAudio.Dmo.AudioMediaSubtypes.MEDIASUBTYPE_IEEE_FLOAT`), trailing partial samples ignored.
 - `FfmpegEncoder.cs:139` pre-existing CS8602 fixed (`process!`) — build **0 warnings**; **139 passing**.
 
-**Deferred:** loopback→encoder mix wiring and the encoder construction (ship step 5); capture while not
-live is deliberately not shipped (privacy indicator otherwise).
+**Deferred:** the loopback→encoder AAC mix wiring (a later step — the encoder construction + frame
+pipeline shipped in ship step 5); capture while not live is deliberately not shipped (privacy indicator
+otherwise).
+
+### Live frame pipeline (TASK 4 ship step 5 — shipped 2026-08-12, plan in TASKS.md)
+
+The **`FramePump`** (`Services/Encoder/`) is the live frame producer: while live it snapshots the active
+scene each tick, resolves every element to its latest frame, composites it into the tier's output frame,
+and paces frames into the encoder at the tier's FPS. **Pattern — everything is a constructor-injected
+seam:** `Func<Scene?>`, `Func<SceneElement, VideoFrame?>` resolver, `Func<CompositorOptions>`,
+`Func<EncoderOptions?>`, `Func<IFfmpegEncoder>`, `Action<string>` log, and an injectable pacing delay
+(default `Task.Delay`; tests inject `Task.Yield`). The pump is free of WPF and of the capture managers.
+
+- **`StartAsync` never throws** — the VM fires-and-forgets it from the sync command handler; failures log
+  + surface via the `Failed` event. `EncoderOptions == null` means "no RTMP URL": the pump logs and skips
+  the encoder entirely. `MainViewModel._rtmpUrlProvider` is that seam — a `Func<string?>` returning null
+  until TASK 5 supplies the reusable stream's ingest URL, so go-live runs the current visual flow.
+- **Stop ordering matters:** `StopAsync` stops the encoder (closes stdin → EOF → ffmpeg finalizes+exits)
+  **before** awaiting the loop, because closing stdin unblocks a write stuck on pipe backpressure — the
+  reverse order would deadlock. `ProcessFailed` self-stops the pump. `HealthUpdated` is forwarded
+  (ship step 6 binds it to the bottom bar); `Failed` while live flips `StreamStatus.Error` (minimal).
+- **`MainViewModel` owns the resolver** (`ResolveOutputFrame`): `WebcamSceneConfig` →
+  `CameraManager.GetLatestFrame(WebcamId)`, `Source { IsLiveCapture, CaptureKey }` →
+  `ScreenCaptureManager.GetLatestFrame(CaptureKey)` (the new accessor mirroring `CameraManager`), image/
+  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.
+- Known consideration: the pump reads the active scene on a background thread while the UI can still edit
+  it; a concurrent-mutation exception is contained (logged + `Failed` + the pump stops) rather than
+  crashing. The background thread + video pipeline is the new reality since this step.
 
 ### Social bar (TASK 14 — shipped 2026-08-12, plan in TASKS.md)