TASK 3 milestone 1: webcam capture (MediaCapture CPU-first, CameraManager refcount, picker, clip/mirror, schema v2, tests)

Services/ICameraEnumerator.cs

@@ -0,0 +1,10 @@
+namespace ytLive.Services;
+
+/// <summary>
+/// Enumerates physical capture devices. Seam so the picker and CameraManager
+/// never touch WinRT directly (tests inject fakes).
+/// </summary>
+public interface ICameraEnumerator
+{
+    Task<IReadOnlyList<CameraDeviceInfo>> GetCamerasAsync();
+}