iOS Safari does not reliably support MediaSource for audio/mpeg streaming,
causing an 'audio decode error' when the MSE SourceBuffer path is used.
Detect iOS (and any browser without audio/mpeg MSE support) at runtime and
fall back to a single fetch(...).blob() download with stream:false. Playback
begins after the full chapter audio is received rather than mid-stream, but
it works on all devices. Desktop browsers keep the existing MSE streaming
path (stream:true) for faster time-to-first-audio.
The same detection applies to the next-chapter prefetch pipeline.
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