Scene compositor (TASK 4 ship step 1): encoder master-frame scaffolding

Software compositor that renders a scene into the encoder's master VideoFrame,
mirroring the XAML preview minus editing chrome: backdrop -> background ->
elements (UniformToFill cover-crop, round clip, mirror, opacity, border) ->
branding flash.

NOTE FOR USERS: this change shows NO difference in the app's UI — it is pure
backend scaffolding laying the groundwork for live video capture/streaming.
The preview you see is unchanged.

- Services/Compositor/: SceneCompositor (Render(scene, frameFor resolver,
  flashFrame, CompositorOptions)), CompositorOptions (source rect + output
  size; 16:9 full master, vertical 607x1080 -> 1080x1920), StretchMath (pure
  UniformToFill + bilinear), StaticPixelCache (asset bytes -> BGRA8 frame)
- Frame sources injected via Func<SceneElement, VideoFrame?> resolver, so the
  compositor is pure, WPF-free, and hermetic to test (D3D11 upgrade behind the
  same seam later)
- SceneElement.TryGetBorderColor public (shared hex parse), stale
  MainViewModel comment fixed, pre-existing CS1998 in YouTubeAuthServiceTests
  cleaned up
- Tests: SceneCompositorTests integration (full scene + vertical tier + flash)
  + StretchMath units, docs updated (72 tests passing, 0 warnings)

Services/index.md

@@ -27,6 +27,10 @@ External-facing logic: YouTube API, persistence. See
 | `ScreenCaptureFrameSource.cs` | One `Direct3D11CaptureFramePool` (free-threaded, 2 buffers) + session per target; frames → `SoftwareBitmap.CreateCopyFromSurfaceAsync` (BGRA, alpha ignored) → `VideoFrame`, bytes read via `WindowsRuntimeMarshal.TryGetDataUnsafe` (CsWinRT-safe — the `IMemoryBufferByteAccess` ComImport cast fails on every frame and is gone). Surfaces > 1920×1080 downscaled bilinearly to the master; conversion failures logged ≤ once/5 s. DRM content = black frames (OS limit). `CreateForMonitor`/`CreateForWindow`/`CreateForPicker` |
 | `ScreenCaptureManager.cs` | Screen-capture ownership mirroring `CameraManager`: refcounted by target key, one shared `WriteableBitmap`, dispatcher-coalesced latest-frame copies; `PreviewBitmapChanged`/`CaptureFailed` events; `ReleaseAllAsync` used on re-designation |
 | `ScreenCaptureSourceFactory.cs` | `Resolve(key)` parses `monitor:<n>` / `window:<hwnd>` / `picker:<name>` into a source; `PickAsync()` shows the OS `GraphicsCapturePicker` and returns the `picker:` key (transient — a reload falls back to auto-detection) |
+| `Compositor/SceneCompositor.cs` | **The output compositor (TASK 4 ship step 1)**: renders a scene into the encoder's master `VideoFrame` (tightly-packed BGRA8), mirroring the XAML preview minus editing chrome — backdrop → background → elements (`UniformToFill` cover-crop, round clip, mirror, opacity, border) → branding flash. Pure and WPF-free: frames injected via a `Func<SceneElement, VideoFrame?>` resolver (webcam → DeviceId, image → AssetId, backdrop → CaptureKey); output sized by `CompositorOptions` (16:9 = full master 1:1; vertical 9:16 = 607×1080 crop → 1080×1920 bilinear). Preview stays XAML (editing view); this is the output view — see `ai.md` "Scene compositor" |
+| `Compositor/CompositorOptions.cs` | The active tier's output rect (source space over the 1920×1080 master, integer-aligned — `MainViewModel.OutputRectX` can be 656.5) + target W×H |
+| `Compositor/StretchMath.cs` | Pure pixel math: the WPF `UniformToFill` cover-crop, clamped bilinear sample/scale (unit-tested half of the compositor) |
+| `Compositor/StaticPixelCache.cs` | Asset bytes → cached BGRA8 `VideoFrame`, decoded once per content hash — the output path's raw-pixel source (the preview uses ImageCache's `BitmapImage`) |
 
 Related: constructed in [`ViewModels/MainViewModel.cs`](../ViewModels/MainViewModel.cs)
 (no DI container yet). Models in [`Models/index.md`](../Models/index.md).