feat(record): fragment the MP4 so a crash cannot destroy the whole take

TASK 32·4. A plain MP4 keeps its index (moov) at the end, so a power cut or a killed
process left the entire recording unreadable even though every video byte was on disk.
The record block now carries
`-movflags +frag_keyframe+empty_moov+default_base_moof`: index first, a playable
fragment flushed at every keyframe, so an interrupted take plays to the last fragment.

Record block only — the FLV stream output has no index and never gets the flags.
Rename-on-stop is untouched.

Test: `Args_RecordBlock_IsFragmentedMp4_SoACrashKeepsTheFile` asserts the flags on the
record block and their absence from the streaming block.

Not yet proven on a real interrupted take (kill ffmpeg mid-recording, play the result);
the args test is by construction.

Docs: task-32 slice 4 ticked with that caveat, TASKS row, output.md record note, HANDOFF.
verify.sh: 550 passed (mouse-injection test excluded, desktop in use), 0 warnings.

ytLive.Tests/FfmpegEncoderTests.cs

@@ -207,6 +207,41 @@ public class FfmpegEncoderTests
         Assert.Equal(@"C:\videos\ty-20260829-1405-0000.mp4", args[idx + 1]);
     }
 
+    /// <summary>TASK 32·4: a recording must survive a crash. A plain MP4 keeps its index at the
+    /// end, so an interrupted file is unreadable — the whole take, not the tail. Fragmented MP4
+    /// writes the index first and flushes a fragment per keyframe, so the file plays to the last
+    /// fragment. The flags belong to the RECORD block only; the FLV stream has no index.</summary>
+    [Fact]
+    public void Args_RecordBlock_IsFragmentedMp4_SoACrashKeepsTheFile()
+    {
+        var recordOnly = FfmpegArgs.Build(
+            new EncoderOptions
+            {
+                StreamEnabled = false,
+                RecordEnabled = true,
+                RecordPath = @"C:\videos\take.mp4",
+                Width = 1920, Height = 1080, Fps = 30, BitrateKbps = 6000,
+            },
+            "h264_nvenc").ToArray();
+
+        var flagsAt = Array.IndexOf(recordOnly, "-movflags");
+        Assert.True(flagsAt > -1, "a recording must be fragmented or a crash destroys the whole file");
+        Assert.Equal("+frag_keyframe+empty_moov+default_base_moof", recordOnly[flagsAt + 1]);
+
+        // …and the stream output must never carry it.
+        var streaming = FfmpegArgs.Build(
+            new EncoderOptions
+            {
+                RtmpUrl = "rtmp://a.rtmp.youtube.com/live2/key",
+                StreamEnabled = true,
+                RecordEnabled = false,
+                Width = 1920, Height = 1080, Fps = 30, BitrateKbps = 6000,
+            },
+            "h264_nvenc").ToArray();
+
+        Assert.DoesNotContain("-movflags", streaming);
+    }
+
     [Fact]
     public void Args_DualOutput_HasTwoMapBlocks_FlvAndMp4()
     {