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)

ytLive.Tests/YouTubeAuthServiceTests.cs

@@ -19,15 +19,15 @@ public class YouTubeAuthServiceTests
             _channelResponse = channelResponse;
         }
 
-        protected override async Task<HttpResponseMessage> SendAsync(
+        protected override Task<HttpResponseMessage> SendAsync(
             HttpRequestMessage request, CancellationToken cancellationToken)
         {
             var isToken = request.RequestUri!.PathAndQuery.Contains("/token");
             var body = isToken ? _tokenResponse : _channelResponse;
-            return new HttpResponseMessage(HttpStatusCode.OK)
+            return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
             {
                 Content = new StringContent(body, Encoding.UTF8, "application/json"),
-            };
+            });
         }
     }