perf(camera): MJPEG negotiation before frame reader — camera drives at its best rate

Slice 10 from today-slices (d1126dd): UVC cameras default MediaFrameReader to their
FIRST media type (YUY2 at crippled fps — C920 720p = 5fps). A 60fps container
then shows a 5-20fps face cam as "every few frames cut out". Fix:
VideoDeviceController.SetMediaStreamPropertiesAsync negotiated to best MJPEG
>=640x360 @>=30fps (<=1280 wide) BEFORE creating the frame reader, with
device-default fallback. Plus a startup.log truth line of the actual agreed media
type so the next diagnosis starts from facts (no more re-deriving the fps).
Build 0 warnings, 289 tests.

Services/MediaCaptureFrameSource.cs

@@ -102,6 +102,43 @@ public sealed class MediaCaptureFrameSource : ICameraFrameSource
             };
             await capture.InitializeAsync(settings);
 
+            // MediaFrameReader defaults to the camera's FIRST media type — on UVC webcams
+            // (Logitech C920 et al.) that is often YUY2 at a crippled rate (720p5!), and a
+            // real-time 60fps container then shows a 5-20fps face cam as "every few frames
+            // cut out". OBS's mechanism, available via VideoDeviceController (VideoProfile
+            // is NOT projected in the 19041 SDK): negotiate the stream property. Pick the
+            // best MJPEG >=640x360 @>=30fps (largest, fastest), set it BEFORE the frame
+            // reader is created, fall back to device default on refusal, and log the live
+            // media type so the next diagnosis starts from facts.
+            try
+            {
+                var controller = capture.VideoDeviceController;
+                var candidates = controller == null
+                    ? new System.Collections.Generic.List<Windows.Media.MediaProperties.VideoEncodingProperties>()
+                    : controller
+                    .GetAvailableMediaStreamProperties(MediaStreamType.VideoPreview)
+                    .OfType<Windows.Media.MediaProperties.VideoEncodingProperties>()
+                    .Where(v => string.Equals(v.Subtype, "MJPG", StringComparison.OrdinalIgnoreCase)
+                                && v.Width >= 640 && v.Height >= 360
+                                && v.FrameRate.Denominator > 0
+                                && v.FrameRate.Numerator / v.FrameRate.Denominator >= 30.0)
+                    .OrderByDescending(v => v.FrameRate.Numerator / (double)Math.Max(1, v.FrameRate.Denominator))
+                    .ThenByDescending(v => (long)v.Width * v.Height)
+                    .ToList();
+                if (candidates.Count > 0)
+                {
+                    var pick = candidates
+                        .OrderBy(v => (long)v.Width * v.Height)
+                        .LastOrDefault(v => v.Width <= 1280) ?? candidates[0];
+                    await controller!.SetMediaStreamPropertiesAsync(MediaStreamType.VideoPreview, pick);
+                }
+            }
+            catch (Exception fmtEx)
+            {
+                AppLog.Write($"MediaCaptureFrameSource: MJPG negotiation refused ({fmtEx.Message}); " +
+                             "keeping device-default media type");
+            }
+
             // Post-init resource validation: the capture must actually be bound to
             // the device we asked for and must expose a live video controller.
             if (!string.Equals(capture.MediaCaptureSettings?.VideoDeviceId, _deviceId,
@@ -147,6 +184,17 @@ public sealed class MediaCaptureFrameSource : ICameraFrameSource
             if (reader == null)
                 throw new InvalidOperationException($"Camera '{_deviceId}' created no frame reader.");
 
+            // One-line truth of what the camera actually agreed to (format/size/fps).
+            try
+            {
+                var active = capture.VideoDeviceController?
+                    .GetMediaStreamProperties(colorSource.Info.MediaStreamType);
+                if (active is Windows.Media.MediaProperties.VideoEncodingProperties vep)
+                    AppLog.Write($"Camera '{_deviceId}' media type: {vep.Subtype} " +
+                                 $"{vep.Width}x{vep.Height} @ {vep.FrameRate.Numerator}/{vep.FrameRate.Denominator}fps");
+            }
+            catch { /* logging must never kill acquisition */ }
+
             reader.FrameArrived += OnFrameArrived;
             var status = await reader.StartAsync();
             if (status != MediaFrameReaderStartStatus.Success)