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/EncoderOptions.cs

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+namespace ytLive.Services.Encoder;
+
+/// <summary>
+/// Everything the FFmpeg subprocess encoder needs for one go-live (TASK 4 ship
+/// step 3). <see cref="RtmpUrl"/> is the FULL ingestion URL — the reusable
+/// stream's ingest address plus its stream key (<c>rtmp://a.rtmp.youtube.com/live2/&lt;key&gt;</c>).
+/// Resolution/FPS/bitrate come from the active quality tier; audio is a silent
+/// placeholder track until the WASAPI capture step replaces the input.
+/// </summary>
+public sealed class EncoderOptions
+{
+    public string RtmpUrl { get; init; } = string.Empty;
+    public int Width { get; init; } = 1920;
+    public int Height { get; init; } = 1080;
+    public int Fps { get; init; } = 60;
+    public int BitrateKbps { get; init; } = 8000;
+
+    /// <summary>H.264 encoder name for <c>-c:v</c>; the encoder probes and prefers
+    /// nvenc → qsv → amf → libopenh264 when not forced.</summary>
+    public string? VideoEncoder { get; init; }
+
+    public int AudioSampleRate { get; init; } = 48000;
+    public int AudioChannels { get; init; } = 2;
+
+    /// <summary>GOP in frames = Fps × 4s — the YouTube keyframe ≤ 4s compliance bound.</summary>
+    public int GopSize => Fps * 4;
+}