Two transcoder fixes for browser MediaSource Extensions parsing:
1. -ar 48000 -ac 2 on the audio output. Source 5.1 / 7.1 streams produced
a moov atom Chrome CHUNK_DEMUXER refuses to parse, even when the video
metadata is fine and a non-MSE video element accepts the same file.
Forcing AAC-LC stereo 48 kHz makes the moov shape MSE-compatible.
2. -frag_duration 1000000 (1 second) so each moof+mdat fragment caps at
~1s of media. Without it, ffmpeg only splits at keyframes and high-
bitrate 1080p produces 8 MiB+ mdat boxes — MSE waits for the whole
mdat before parsing the first fragment, so playback never starts.
3. -movflags +negative_cts_offsets so b-frames carry the right pts/dts
offsets and the playhead doesn't reset every fragment.
4. New range_req debug log to make sizing bugs greppable.
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