TASK 4 ship step 3: encoder + RTMP push — FFmpeg subprocess with probed H.264 picker, BGRA stdin feed, stderr health parsing, graceful stop — 122 tests passing, 0 warnings

Services/Encoder/IFfmpegEncoder.cs

@@ -0,0 +1,25 @@
+using ytLive.Models;
+
+namespace ytLive.Services.Encoder;
+
+/// <summary>
+/// The live encoder seam (TASK 4 ship step 3): start an FFmpeg subprocess that
+/// encodes raw BGRA frames from stdin and pushes FLV to an RTMP ingestion URL,
+/// raising parsed health stats from stderr. The frame producer (compositor →
+/// capture managers, TASK 4 ship step 5) feeds <see cref="SubmitFrameAsync"/> at
+/// capture rate; this service serializes writes, parses progress, and tears the
+/// process down gracefully. Constructor-injected <see cref="IFfmpegLocator"/> +
+/// process factory keep it hermetic (tests fake both).
+/// </summary>
+public interface IFfmpegEncoder : IDisposable
+{
+    /// <summary>Fires on each parsed ffmpeg <c>-stats</c> progress line (~2 Hz).</summary>
+    event EventHandler<StreamHealth>? HealthUpdated;
+
+    /// <summary>Fires when the subprocess dies unexpectedly (non-zero exit while live).</summary>
+    event EventHandler<string>? ProcessFailed;
+
+    Task StartAsync(EncoderOptions options, CancellationToken cancellationToken = default);
+    Task SubmitFrameAsync(VideoFrame frame, CancellationToken cancellationToken = default);
+    Task StopAsync(CancellationToken cancellationToken = default);
+}