18a21010bb
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)