Measured: on a quiet sink the segment muxer held the Opus pages in a buffer; a
segment was still 0 bytes after 25 s. A kill or a power cut would lose the quiet
stretch, and 'is the file growing' could not tell a healthy recorder from a dead
one (the health check cried 'rig is down' on a working recorder). With
segment_format_options flush_packets=1 the file grows every second, in silence
too.