feat(65): the clip mux — a third output on the same ffmpeg, drained off the frame path

Slice 2 of TASK 65. 602 green, 0 warnings. **Clips are still off**: no `ClipBuffer` is passed, so the
ffmpeg command line is byte-for-byte what it was before this commit, and a test asserts exactly that
(the original args are a strict prefix of the new ones).

- `FfmpegArgs` appends a fragmented-MP4 output to `pipe:1` only when `ClipBuffer` is set. One-second
  fragments (`-frag_duration 1000000`) rather than the 4 s GOP, so a clip can start inside the window
  instead of at the last keyframe. Stats already own stderr, so stdout was free.
- `Fmp4FragmentSplitter` turns that byte stream into whole fragments: `ftyp`+`moov` is the init
  segment, each `moof`+`mdat` is a fragment. It parses box headers itself, survives a box being cut
  anywhere across reads, and **resyncs instead of throwing** — a clip fault must never take the stream
  down.
- `FfmpegEncoder` reads it on its own task, never blocking the frame path. Any exception logs and
  disables clips for the session; the broadcast is untouched.
- `IEncoderProcess` gained `StandardOutputBytes` (the seam already had the text reader for probing).

Next slices: the replay layer + retention, then the Settings switch with disabled-but-remembered controls.

Services/Encoder/FfmpegArgs.cs

@@ -52,6 +52,19 @@ public static class FfmpegArgs
             args.AddRange(["-f", "mp4", options.RecordPath!]);
         }
 
+        // TASK 65: the replay buffer is a third mux of the same encoded frames. Fragmented MP4 on
+        // stdout (stats already own stderr) — one fragment per second, so a clip can start anywhere
+        // inside the chosen window instead of at the last 4-second keyframe.
+        if (options.ClipBuffer != null)
+        {
+            AddVideoTags(args, options, videoEncoder);
+            args.AddRange(["-c:a", "aac",
+                "-ar", options.AudioSampleRate.ToString(), "-ac", options.AudioChannels.ToString()]);
+            args.AddRange(["-movflags", "+frag_keyframe+empty_moov+default_base_moof"]);
+            args.AddRange(["-frag_duration", "1000000"]);
+            args.AddRange(["-f", "mp4", "pipe:1"]);
+        }
+
         return args;
     }