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/Compositor/CompositorOptions.cs

@@ -0,0 +1,19 @@
+namespace ytLive.Services.Compositor;
+
+/// <summary>
+/// Where the compositor draws from (the active quality tier's output rect over the
+/// 1920×1080 master) and at what size. 16:9 tiers = the full master 1:1; the vertical
+/// 9:16 tier = the centered 607×1080 crop scaled up to 1080×1920. The source rect is
+/// integer-aligned — MainViewModel's <c>OutputRectX</c> can be 656.5, so the caller
+/// rounds before building these options.
+/// </summary>
+public sealed class CompositorOptions
+{
+    public int SourceRectX { get; init; }
+    public int SourceRectY { get; init; }
+    public int SourceRectWidth { get; init; }
+    public int SourceRectHeight { get; init; }
+
+    public int OutputWidth { get; init; }
+    public int OutputHeight { get; init; }
+}